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

plans/time-decay-expert-scoring.md

@@ -4,7 +4,7 @@ title: ""
 status: iterating
 iteration: 6
 target_iterations: 8
-beads_revision: 1
+beads_revision: 2
 related_plans: []
 created: 2026-02-08
 updated: 2026-02-12