docs: add feature ideas catalog, time-decay scoring plan, and timeline issue doc

Ideas catalog (docs/ideas/): 25 feature concept documents covering future
lore capabilities including bottleneck detection, churn analysis, expert
scoring, collaboration patterns, milestone risk, knowledge silos, and more.
Each doc includes motivation, implementation sketch, data requirements, and
dependencies on existing infrastructure. README.md provides an overview and
SYSTEM-PROPOSAL.md presents the unified analytics vision.

Plans (plans/): Time-decay expert scoring design with four rounds of review
feedback exploring decay functions, scoring algebra, and integration points
with the existing who-expert pipeline.

Issue doc (docs/issues/001): Documents the timeline pipeline bug where
EntityRef was missing project context, causing ambiguous cross-project
references during the EXPAND stage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plans/time-decay-expert-scoring.feedback-1.md

@@ -0,0 +1,114 @@
+Your plan is strong directionally, but I’d revise it in 8 key places to avoid regressions and make it significantly more useful in production.
+
+1. **Split reviewer signals into “participated” vs “assigned-only”**
+Reason: today’s inflation problem is often assignment noise. Treating `mr_reviewers` equal to real review activity still over-ranks passive reviewers.
+
+```diff
+@@ Per-signal contributions
+-| Reviewer (reviewed MR touching path) | 10 | 90 days |
++| ReviewerParticipated (left DiffNote on MR/path) | 10 | 90 days |
++| ReviewerAssignedOnly (in mr_reviewers, no DiffNote by that user on MR/path) | 3 | 45 days |
+```
+
+```diff
+@@ Scoring Formula
+-score = reviewer_mr * reviewer_weight + ...
++score = reviewer_participated * reviewer_weight
++      + reviewer_assigned_only * reviewer_assignment_weight
++      + ...
+```
+
+2. **Cap/saturate note intensity per MR**
+Reason: raw per-note addition can still reward “comment storms.” Use diminishing returns.
+
+```diff
+@@ Rust-Side Aggregation
+-- Notes: Vec<i64> (timestamps) from diffnote_reviewer
++-- Notes grouped per (username, mr_id): note_count + max_ts
++-- Note contribution per MR uses diminishing returns:
++-- note_score_mr = note_bonus * ln(1 + note_count) * decay(now - ts, note_hl)
+```
+
+3. **Use better event timestamps than `m.updated_at` for file-change signals**
+Reason: `updated_at` is noisy (title edits, metadata touches) and creates false recency.
+
+```diff
+@@ SQL Restructure
+- signal 3/4 seen_at = m.updated_at
++ signal 3/4 activity_ts = COALESCE(m.merged_at, m.closed_at, m.created_at, m.updated_at)
+```
+
+4. **Don’t stream raw note rows to Rust; pre-aggregate in SQL**
+Reason: current plan removes SQL grouping and can blow up memory/latency on large repos.
+
+```diff
+@@ SQL Restructure
+-SELECT username, signal, mr_id, note_id, ts FROM signals
++WITH raw_signals AS (...),
++aggregated AS (
++  -- 1 row per (username, signal_class, mr_id) for MR-level signals
++  -- 1 row per (username, mr_id) for note_count + max_ts
++)
++SELECT username, signal_class, mr_id, qty, ts FROM aggregated
+```
+
+5. **Replace fixed `"24m"` with model-driven cutoff**
+Reason: hardcoded 24m is arbitrary and tied to current weights/half-lives only.
+
+```diff
+@@ Default --since Change
+-Expert mode: "6m" -> "24m"
++Expert mode default window derived from scoring.max_age_days (default 1095 days / 36m).
++Formula guidance: choose max_age where max possible single-event contribution < epsilon (e.g. 0.25 points).
++Add `--all-history` to disable cutoff for diagnostics.
+```
+
+6. **Validate scoring config explicitly**
+Reason: silent bad configs (`half_life_days = 0`, negative weights) create undefined behavior.
+
+```diff
+@@ ScoringConfig (config.rs)
+ pub struct ScoringConfig {
+   pub author_weight: i64,
+   pub reviewer_weight: i64,
+   pub note_bonus: i64,
++  pub reviewer_assignment_weight: i64,   // default: 3
+   pub author_half_life_days: u32,
+   pub reviewer_half_life_days: u32,
+   pub note_half_life_days: u32,
++  pub reviewer_assignment_half_life_days: u32, // default: 45
++  pub max_age_days: u32, // default: 1095
+ }
+@@ Config::load_from_path
++validate_scoring(&config.scoring)?; // weights >= 0, half_life_days > 0, max_age_days >= 30
+```
+
+7. **Keep raw float score internally; round only for display**
+Reason: rounding before sort causes avoidable ties/rank instability.
+
+```diff
+@@ Rust-Side Aggregation
+-Round to i64 for Expert.score field
++Compute `raw_score: f64`, sort by raw_score DESC.
++Expose integer `score` for existing UX.
++Optionally expose `score_raw` and `score_components` in robot JSON when `--explain-score`.
+```
+
+8. **Add confidence + data-completeness metadata**
+Reason: rankings are misleading if `mr_file_changes` coverage is poor.
+
+```diff
+@@ ExpertResult / Output
++confidence: "high" | "medium" | "low"
++coverage: { mrs_with_file_changes, total_mrs_in_window, percent }
++warning when coverage < threshold (e.g. 70%)
+```
+
+```diff
+@@ Verification
+ 4. cargo test
++5. ubs src/cli/commands/who.rs src/core/config.rs
++6. Benchmark query_expert on representative DB (latency + rows scanned before/after)
+```
+
+If you want, I can rewrite your full plan document into a clean “v2” version that already incorporates these diffs end-to-end.