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-2.md

@@ -0,0 +1,132 @@
+The plan is strong, but I’d revise it in 10 places to improve correctness, scalability, and operator trust.
+
+1. **Add rename/old-path awareness (correctness gap)**
+Analysis: right now both existing code and your plan still center on `position_new_path` / `new_path` matches (`src/cli/commands/who.rs:643`, `src/cli/commands/who.rs:681`). That misses expertise on renamed/deleted paths and under-ranks long-time owners after refactors.
+
+```diff
+@@ ## Context
+-This produces two compounding problems:
++This produces three compounding problems:
+@@
+ 2. **Reviewer inflation**: ...
++3. **Path-history blindness**: Renamed/moved files lose historical expertise because matching relies on current-path fields only.
+
+@@ ### 3. SQL Restructure (who.rs)
+-AND n.position_new_path {path_op}
++AND (n.position_new_path {path_op} OR n.position_old_path {path_op})
+
+-AND fc.new_path {path_op}
++AND (fc.new_path {path_op} OR fc.old_path {path_op})
+```
+
+2. **Follow rename chains for queried paths**
+Analysis: matching `old_path` helps, but true continuity needs alias expansion (A→B→C). Without this, expertise before multi-hop renames is fragmented.
+
+```diff
+@@ ### 3. SQL Restructure (who.rs)
++**Path alias expansion**: Before scoring, resolve a bounded rename alias set (default max depth: 20)
++from `mr_file_changes(change_type='renamed')`. Query signals against all aliases.
++Output includes `path_aliases_used` for transparency.
+```
+
+3. **Use hybrid SQL pre-aggregation instead of fully raw rows**
+Analysis: the “raw row” design is simpler but will degrade on large repos with heavy DiffNote volume. Pre-aggregating to `(user, mr)` for MR signals and `(user, mr, note_count)` for note signals keeps memory/latency predictable.
+
+```diff
+@@ ### 3. SQL Restructure (who.rs)
+-The SQL CTE ... removes the outer GROUP BY aggregation. Instead, it returns raw signal rows:
+-SELECT username, signal, mr_id, note_id, ts FROM signals
++Use hybrid aggregation:
++- SQL returns MR-level rows for author/reviewer signals (1 row per user+MR+signal_class)
++- SQL returns note groups (1 row per user+MR with note_count, max_ts)
++- Rust applies decay + ln(1+count) + final ranking.
+```
+
+4. **Make timestamp policy state-aware (merged vs opened)**
+Analysis: replacing `updated_at` with only `COALESCE(merged_at, created_at)` over-decays long-running open MRs. Open MRs need recency from active lifecycle; merged MRs should anchor to merge time.
+
+```diff
+@@ ### 3. SQL Restructure (who.rs)
+-Replace m.updated_at with COALESCE(m.merged_at, m.created_at)
++Use state-aware timestamp:
++activity_ts =
++  CASE
++    WHEN m.state = 'merged' THEN COALESCE(m.merged_at, m.updated_at, m.created_at, m.last_seen_at)
++    WHEN m.state = 'opened' THEN COALESCE(m.updated_at, m.created_at, m.last_seen_at)
++  END
+```
+
+5. **Replace fixed `24m` with config-driven max age**
+Analysis: `24m` is reasonable now, but brittle after tuning weights/half-lives. Tie cutoff to config so model behavior remains coherent as parameters evolve.
+
+```diff
+@@ ### 1. ScoringConfig (config.rs)
++pub max_age_days: u32,                  // default: 730 (or 1095)
+
+@@ ### 5. Default --since Change
+-Expert mode: "6m" -> "24m"
++Expert mode default window derives from `scoring.max_age_days`
++unless user passes `--since` or `--all-history`.
+```
+
+6. **Add reproducible scoring time via `--as-of`**
+Analysis: decayed ranking is time-sensitive; debugging and tests become flaky without a fixed reference clock. This improves reliability and incident triage.
+
+```diff
+@@ ## Files to Modify
+-2. src/cli/commands/who.rs
++2. src/cli/commands/who.rs
++3. src/cli/mod.rs
++4. src/main.rs
+
+@@ ### 5. Default --since Change
++Add `--as-of <RFC3339|YYYY-MM-DD>` to score at a fixed timestamp.
++`resolved_input` includes `as_of_ms` and `as_of_iso`.
+```
+
+7. **Add explainability output (`--explain-score`)**
+Analysis: decayed multi-signal ranking will be disputed without decomposition. Show components and top evidence MRs to make results actionable and debuggable.
+
+```diff
+@@ ## Rejected Ideas (with rationale)
+-- **`--explain-score` flag with component breakdown**: ... deferred
++**Included in this iteration**: `--explain-score` adds per-user score components
++(`author`, `review_participated`, `review_assigned`, `notes`) plus top evidence MRs.
+```
+
+8. **Add confidence/coverage metadata**
+Analysis: rankings can look precise while data is incomplete (`mr_file_changes` gaps, sparse DiffNotes). Confidence fields prevent false certainty.
+
+```diff
+@@ ### 4. Rust-Side Aggregation (who.rs)
++Compute and emit:
++- `coverage`: {mrs_considered, mrs_with_file_changes, mrs_with_diffnotes, percent}
++- `confidence`: high|medium|low (threshold-based)
+```
+
+9. **Add index migration for new query shapes**
+Analysis: your new `EXISTS/NOT EXISTS` reviewer split and path dual-matching will need better indexes; current `who` indexes are not enough for author+path+time combinations.
+
+```diff
+@@ ## Files to Modify
++3. **`migrations/021_who_decay_indexes.sql`** — indexes for decay query patterns:
++   - notes(diffnote path + author + created_at + discussion_id) partial
++   - notes(old_path variant) partial
++   - mr_file_changes(project_id, new_path, merge_request_id)
++   - mr_file_changes(project_id, old_path, merge_request_id) partial
++   - merge_requests(state, merged_at, updated_at, created_at)
+```
+
+10. **Expand tests to invariants and determinism**
+Analysis: example-based tests are good, but ranking systems need invariant tests to avoid subtle regressions.
+
+```diff
+@@ ### 7. New Tests (TDD)
++**`test_score_monotonicity_by_age`**: same signal, older timestamp never scores higher
++**`test_row_order_independence`**: shuffled SQL row order yields identical ranking
++**`test_as_of_reproducibility`**: same data + same `--as-of` => identical output
++**`test_rename_alias_chain_scoring`**: expertise carries across A->B->C rename chain
++**`test_overlap_participated_vs_assigned_counts`**: overlap reflects split reviewer semantics
+```
+
+If you want, I can produce a full consolidated `v2` plan doc patch (single unified diff against `plans/time-decay-expert-scoring.md`) rather than per-change snippets.