feat(bd-226s): implement time-decay expert scoring model

Replace flat-weight expertise scoring with exponential half-life decay,
split reviewer signals (participated vs assigned-only), dual-path rename
awareness, and new CLI flags (--as-of, --explain-score, --include-bots,
--all-history).

Changes:
- ScoringConfig: 8 new fields with validation (config.rs)
- half_life_decay() and normalize_query_path() pure functions (who.rs)
- CTE-based SQL with dual-path matching, mr_activity, reviewer_participation (who.rs)
- Rust-side decay aggregation with deterministic f64 ordering (who.rs)
- Path resolution probes check old_path columns (who.rs)
- Migration 026: 5 new indexes for dual-path and reviewer participation
- Default --since changed from 6m to 24m
- 31 new tests (example-based + invariant), 621 total who tests passing
- Autocorrect registry updated with new flags

Closes: bd-226s, bd-2w1p, bd-1soz, bd-18dn, bd-2ao4, bd-2yu5, bd-1b50,
bd-1hoq, bd-1h3f, bd-13q8, bd-11mg, bd-1vti, bd-1j5o

src/core/config.rs

@@ -164,6 +164,38 @@ pub struct ScoringConfig {
     /// Bonus points per individual inline review comment (DiffNote).
     #[serde(rename = "noteBonus")]
     pub note_bonus: i64,
+
+    /// Points per MR where the user was assigned as a reviewer.
+    #[serde(rename = "reviewerAssignmentWeight")]
+    pub reviewer_assignment_weight: i64,
+
+    /// Half-life in days for author contribution decay.
+    #[serde(rename = "authorHalfLifeDays")]
+    pub author_half_life_days: u32,
+
+    /// Half-life in days for reviewer contribution decay.
+    #[serde(rename = "reviewerHalfLifeDays")]
+    pub reviewer_half_life_days: u32,
+
+    /// Half-life in days for reviewer assignment decay.
+    #[serde(rename = "reviewerAssignmentHalfLifeDays")]
+    pub reviewer_assignment_half_life_days: u32,
+
+    /// Half-life in days for note/comment contribution decay.
+    #[serde(rename = "noteHalfLifeDays")]
+    pub note_half_life_days: u32,
+
+    /// Multiplier applied to scores from closed (not merged) MRs.
+    #[serde(rename = "closedMrMultiplier")]
+    pub closed_mr_multiplier: f64,
+
+    /// Minimum character count for a review note to earn note_bonus.
+    #[serde(rename = "reviewerMinNoteChars")]
+    pub reviewer_min_note_chars: u32,
+
+    /// Usernames excluded from expert/scoring results.
+    #[serde(rename = "excludedUsernames")]
+    pub excluded_usernames: Vec<String>,
 }
 
 impl Default for ScoringConfig {
@@ -172,6 +204,14 @@ impl Default for ScoringConfig {
             author_weight: 25,
             reviewer_weight: 10,
             note_bonus: 1,
+            reviewer_assignment_weight: 3,
+            author_half_life_days: 180,
+            reviewer_half_life_days: 90,
+            reviewer_assignment_half_life_days: 45,
+            note_half_life_days: 45,
+            closed_mr_multiplier: 0.5,
+            reviewer_min_note_chars: 20,
+            excluded_usernames: vec![],
         }
     }
 }
@@ -287,6 +327,55 @@ fn validate_scoring(scoring: &ScoringConfig) -> Result<()> {
             details: "scoring.noteBonus must be >= 0".to_string(),
         });
     }
+    if scoring.reviewer_assignment_weight < 0 {
+        return Err(LoreError::ConfigInvalid {
+            details: "scoring.reviewerAssignmentWeight must be >= 0".to_string(),
+        });
+    }
+    if scoring.author_half_life_days == 0 || scoring.author_half_life_days > 3650 {
+        return Err(LoreError::ConfigInvalid {
+            details: "scoring.authorHalfLifeDays must be in 1..=3650".to_string(),
+        });
+    }
+    if scoring.reviewer_half_life_days == 0 || scoring.reviewer_half_life_days > 3650 {
+        return Err(LoreError::ConfigInvalid {
+            details: "scoring.reviewerHalfLifeDays must be in 1..=3650".to_string(),
+        });
+    }
+    if scoring.reviewer_assignment_half_life_days == 0
+        || scoring.reviewer_assignment_half_life_days > 3650
+    {
+        return Err(LoreError::ConfigInvalid {
+            details: "scoring.reviewerAssignmentHalfLifeDays must be in 1..=3650".to_string(),
+        });
+    }
+    if scoring.note_half_life_days == 0 || scoring.note_half_life_days > 3650 {
+        return Err(LoreError::ConfigInvalid {
+            details: "scoring.noteHalfLifeDays must be in 1..=3650".to_string(),
+        });
+    }
+    if !scoring.closed_mr_multiplier.is_finite()
+        || scoring.closed_mr_multiplier <= 0.0
+        || scoring.closed_mr_multiplier > 1.0
+    {
+        return Err(LoreError::ConfigInvalid {
+            details: "scoring.closedMrMultiplier must be finite and in (0.0, 1.0]".to_string(),
+        });
+    }
+    if scoring.reviewer_min_note_chars > 4096 {
+        return Err(LoreError::ConfigInvalid {
+            details: "scoring.reviewerMinNoteChars must be <= 4096".to_string(),
+        });
+    }
+    if scoring
+        .excluded_usernames
+        .iter()
+        .any(|u| u.trim().is_empty())
+    {
+        return Err(LoreError::ConfigInvalid {
+            details: "scoring.excludedUsernames entries must be non-empty".to_string(),
+        });
+    }
     Ok(())
 }
 
@@ -561,4 +650,140 @@ mod tests {
             "set default_project should be present: {json}"
         );
     }
+
+    #[test]
+    fn test_config_validation_rejects_zero_half_life() {
+        let scoring = ScoringConfig {
+            author_half_life_days: 0,
+            ..Default::default()
+        };
+        let err = validate_scoring(&scoring).unwrap_err();
+        let msg = err.to_string();
+        assert!(
+            msg.contains("authorHalfLifeDays"),
+            "unexpected error: {msg}"
+        );
+    }
+
+    #[test]
+    fn test_config_validation_rejects_absurd_half_life() {
+        let scoring = ScoringConfig {
+            author_half_life_days: 5000,
+            ..Default::default()
+        };
+        let err = validate_scoring(&scoring).unwrap_err();
+        let msg = err.to_string();
+        assert!(
+            msg.contains("authorHalfLifeDays"),
+            "unexpected error: {msg}"
+        );
+    }
+
+    #[test]
+    fn test_config_validation_rejects_nan_multiplier() {
+        let scoring = ScoringConfig {
+            closed_mr_multiplier: f64::NAN,
+            ..Default::default()
+        };
+        let err = validate_scoring(&scoring).unwrap_err();
+        let msg = err.to_string();
+        assert!(
+            msg.contains("closedMrMultiplier"),
+            "unexpected error: {msg}"
+        );
+    }
+
+    #[test]
+    fn test_config_validation_rejects_zero_multiplier() {
+        let scoring = ScoringConfig {
+            closed_mr_multiplier: 0.0,
+            ..Default::default()
+        };
+        let err = validate_scoring(&scoring).unwrap_err();
+        let msg = err.to_string();
+        assert!(
+            msg.contains("closedMrMultiplier"),
+            "unexpected error: {msg}"
+        );
+    }
+
+    #[test]
+    fn test_config_validation_rejects_negative_reviewer_assignment_weight() {
+        let scoring = ScoringConfig {
+            reviewer_assignment_weight: -1,
+            ..Default::default()
+        };
+        let err = validate_scoring(&scoring).unwrap_err();
+        let msg = err.to_string();
+        assert!(
+            msg.contains("reviewerAssignmentWeight"),
+            "unexpected error: {msg}"
+        );
+    }
+
+    #[test]
+    fn test_config_validation_rejects_oversized_min_note_chars() {
+        let scoring = ScoringConfig {
+            reviewer_min_note_chars: 5000,
+            ..Default::default()
+        };
+        let err = validate_scoring(&scoring).unwrap_err();
+        let msg = err.to_string();
+        assert!(
+            msg.contains("reviewerMinNoteChars"),
+            "unexpected error: {msg}"
+        );
+    }
+
+    #[test]
+    fn test_config_validation_rejects_empty_excluded_username() {
+        let scoring = ScoringConfig {
+            excluded_usernames: vec!["valid".to_string(), "  ".to_string()],
+            ..Default::default()
+        };
+        let err = validate_scoring(&scoring).unwrap_err();
+        let msg = err.to_string();
+        assert!(msg.contains("excludedUsernames"), "unexpected error: {msg}");
+    }
+
+    #[test]
+    fn test_config_validation_accepts_valid_new_fields() {
+        let scoring = ScoringConfig {
+            author_half_life_days: 365,
+            reviewer_half_life_days: 180,
+            reviewer_assignment_half_life_days: 90,
+            note_half_life_days: 60,
+            closed_mr_multiplier: 0.5,
+            reviewer_min_note_chars: 20,
+            reviewer_assignment_weight: 3,
+            excluded_usernames: vec!["bot-user".to_string()],
+            ..Default::default()
+        };
+        validate_scoring(&scoring).unwrap();
+    }
+
+    #[test]
+    fn test_config_validation_accepts_boundary_half_life() {
+        // 1 and 3650 are both valid boundaries
+        let scoring_min = ScoringConfig {
+            author_half_life_days: 1,
+            ..Default::default()
+        };
+        validate_scoring(&scoring_min).unwrap();
+
+        let scoring_max = ScoringConfig {
+            author_half_life_days: 3650,
+            ..Default::default()
+        };
+        validate_scoring(&scoring_max).unwrap();
+    }
+
+    #[test]
+    fn test_config_validation_accepts_multiplier_at_one() {
+        let scoring = ScoringConfig {
+            closed_mr_multiplier: 1.0,
+            ..Default::default()
+        };
+        validate_scoring(&scoring).unwrap();
+    }
 }

src/core/db.rs

@@ -85,6 +85,10 @@ const MIGRATIONS: &[(&str, &str)] = &[
         "025",
         include_str!("../../migrations/025_note_dirty_backfill.sql"),
     ),
+    (
+        "026",
+        include_str!("../../migrations/026_scoring_indexes.sql"),
+    ),
 ];
 
 pub fn create_connection(db_path: &Path) -> Result<Connection> {