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

migrations/026_scoring_indexes.sql

@@ -0,0 +1,20 @@
+-- Indexes for time-decay expert scoring: dual-path matching and reviewer participation.
+
+CREATE INDEX IF NOT EXISTS idx_notes_old_path_author
+  ON notes(position_old_path, author_username, created_at)
+  WHERE note_type = 'DiffNote' AND is_system = 0 AND position_old_path IS NOT NULL;
+
+CREATE INDEX IF NOT EXISTS idx_mfc_old_path_project_mr
+  ON mr_file_changes(old_path, project_id, merge_request_id)
+  WHERE old_path IS NOT NULL;
+
+CREATE INDEX IF NOT EXISTS idx_mfc_new_path_project_mr
+  ON mr_file_changes(new_path, project_id, merge_request_id);
+
+CREATE INDEX IF NOT EXISTS idx_notes_diffnote_discussion_author
+  ON notes(discussion_id, author_username, created_at)
+  WHERE note_type = 'DiffNote' AND is_system = 0;
+
+CREATE INDEX IF NOT EXISTS idx_notes_old_path_project_created
+  ON notes(position_old_path, project_id, created_at)
+  WHERE note_type = 'DiffNote' AND is_system = 0 AND position_old_path IS NOT NULL;