feat: Implement Gate 3 timeline pipeline and Gate 4 migration scaffolding

Complete 5 beads for the Phase B temporal intelligence feature:

- bd-1oo: Register migration 015 (commit SHAs, closes watermark) and
  create migration 016 (mr_file_changes table with 4 indexes for
  Gate 4 file-history)

- bd-20e: Define TimelineEvent model with 9 event type variants,
  EntityRef, ExpandedEntityRef, UnresolvedRef, and TimelineResult
  types. Ord impl for chronological sorting with stable tiebreak.

- bd-32q: Implement timeline seed phase - FTS5 keyword search to
  entity IDs with discussion-to-parent resolution, entity dedup,
  and evidence note extraction with snippet truncation.

- bd-ypa: Implement timeline expand phase - BFS cross-reference
  expansion over entity_references with bidirectional traversal,
  depth limiting, mention filtering, provenance tracking, and
  unresolved reference collection.

- bd-3as: Implement timeline event collection - gathers Created,
  StateChanged, LabelAdded/Removed, MilestoneSet/Removed, Merged,
  and NoteEvidence events. Merged dedup (state=merged -> Merged
  variant only). NULL label/milestone fallbacks. Chronological
  interleaving with since filter and limit.

38 new tests, all 445 tests pass. All quality gates clean.

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

src/core/db.rs

@@ -44,6 +44,14 @@ const MIGRATIONS: &[(&str, &str)] = &[
         "014",
         include_str!("../../migrations/014_sync_runs_enrichment.sql"),
     ),
+    (
+        "015",
+        include_str!("../../migrations/015_commit_shas_and_closes_watermark.sql"),
+    ),
+    (
+        "016",
+        include_str!("../../migrations/016_mr_file_changes.sql"),
+    ),
 ];
 
 pub fn create_connection(db_path: &Path) -> Result<Connection> {