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/mod.rs

@@ -14,6 +14,10 @@ pub mod project;
 pub mod references;
 pub mod sync_run;
 pub mod time;
+pub mod timeline;
+pub mod timeline_collect;
+pub mod timeline_expand;
+pub mod timeline_seed;
 
 pub use config::Config;
 pub use error::{LoreError, Result};