docs: add Phase 4 implementation plan (vector retrieval + branching + polish)

15 tasks across 8 waves landing the Phase 4 deliverables per
requirements doc §13 + §14:

- Vector retrieval via sqlite-vec (new external dependency)
- Branching UI (event log forks)
- Drawer-edit on every field (significance review, hide-from-view,
  surgical delete with cascade preview, branching affordances)
- Backup tooling (snapshot UX surface)
- Cross-chat search

Plus the 3 Phase 3.6 carry-over fixes (T90 bundle).

Wave structure:
- W1 (parallel 3-way): schema foundation + carry-overs
- W2 (parallel 3-way): embedding/search services
- W3 (parallel 2-way): branching + delete services
- W4 (single): combined retrieval ranking
- W5 (single): memory write hook + backfill
- W6 (single): drawer Phase 4 bundle (5 sub-features)
- W7 (parallel 2-way): snapshot UX + cross-chat search UX
- W8 (parallel 2-way): integration tests + docs

External dependency: sqlite-vec must be installed BEFORE Wave 1.
Embedding model choice (384-dim default) pinned in T91 before dispatch
since the migration hardcodes the dimension.

Schema baseline: 11 -> 13 (adds 0012_embeddings.sql + 0013_branches.sql).
Task ids T88-T102 to avoid collision with prior phases.

docs/plans/2026-04-27-v4-phase4-implementation.md.tasks.json

@@ -0,0 +1,22 @@
+{
+  "planPath": "docs/plans/2026-04-27-v4-phase4-implementation.md",
+  "tasks": [
+    {"id": 88, "subject": "T88: embeddings table + projector handlers (sqlite-vec)", "status": "pending", "wave": 1, "parallelGroup": "wave-1"},
+    {"id": 89, "subject": "T89: branches table + projector handlers", "status": "pending", "wave": 1, "parallelGroup": "wave-1"},
+    {"id": 90, "subject": "T90: phase 3.6 carry-overs (chat-id pushdown + lifecycle wording + legacy fn consolidation)", "status": "pending", "wave": 1, "parallelGroup": "wave-1"},
+    {"id": 91, "subject": "T91: embedding generation service", "status": "pending", "wave": 2, "parallelGroup": "wave-2", "blockedBy": [88]},
+    {"id": 92, "subject": "T92: vector search service via sqlite-vec", "status": "pending", "wave": 2, "parallelGroup": "wave-2", "blockedBy": [88]},
+    {"id": 93, "subject": "T93: cross-chat search service (FTS5 over all owners)", "status": "pending", "wave": 2, "parallelGroup": "wave-2"},
+    {"id": 94, "subject": "T94: branch_from_event service", "status": "pending", "wave": 3, "parallelGroup": "wave-3", "blockedBy": [89]},
+    {"id": 95, "subject": "T95: delete-impact computation service", "status": "pending", "wave": 3, "parallelGroup": "wave-3"},
+    {"id": 96, "subject": "T96: combined FTS + vector retrieval ranking in search_memories", "status": "pending", "wave": 4, "parallelGroup": null, "blockedBy": [91, 92]},
+    {"id": 97, "subject": "T97: memory_write enqueues embedding job + backfill script", "status": "pending", "wave": 5, "parallelGroup": null, "blockedBy": [91, 96]},
+    {"id": 98, "subject": "T98: drawer Phase 4 bundle (branching + sig review + hide + surgical delete + remaining edits)", "status": "pending", "wave": 6, "parallelGroup": null, "blockedBy": [94, 95, 97]},
+    {"id": 99, "subject": "T99: snapshot UX (manual trigger + list + restore + preview)", "status": "pending", "wave": 7, "parallelGroup": "wave-7"},
+    {"id": 100, "subject": "T100: cross-chat search UX (top-bar + results page)", "status": "pending", "wave": 7, "parallelGroup": "wave-7", "blockedBy": [93]},
+    {"id": 101, "subject": "T101: cross-feature integration tests (vector × branching × delete × snapshot × search)", "status": "pending", "wave": 8, "parallelGroup": "wave-8", "blockedBy": [98, 99, 100]},
+    {"id": 102, "subject": "T102: Phase 4 documentation update", "status": "pending", "wave": 8, "parallelGroup": "wave-8", "blockedBy": [98, 99, 100]}
+  ],
+  "lastUpdated": "2026-04-27T00:00:00Z",
+  "notes": "15 tasks across 8 waves. Adds vector retrieval (sqlite-vec), branching UI, drawer-edit on every field, backup tooling, significance review UI, surgical delete with cascade preview, hide-from-view, and cross-chat search. Phase 3.6 carry-overs (3 small fixes) bundled into T90. External dependency: sqlite-vec must be installed BEFORE Wave 1 dispatch. Embedding model choice (default: 384-dim small model) pinned in T91 spec before dispatch — schema 0012 hardcodes 384 dim. Two new schema migrations (0012 embeddings, 0013 branches), final schema version 13. Uses task ids T88-T102."
+}