feat(pilot): Phase 2 — What we know (facts) with stable task-lane IDs

Adds the load-bearing structural feature of the FlowPilot migration: a
"What we know" panel that holds confirmed facts for a session, fed by AI
[PROMOTE] markers and engineer-added notes. Facts feed the resolution
note preview (Phase 3) and survive across turns via stable UUIDs assigned
to pending_task_lane items.

Backend:
- FactSynthesisService: create/update/soft-delete facts with atomic
  state_version bumps; LLM-backed synthesize_from_question/check on the
  fact_synthesis (Haiku) action tier per Section 6.6.
- /api/v1/ai-sessions/{id}/facts CRUD + /facts/promote (proposed_text or
  via synthesis). PATCH returns 403 for question/diagnostic_check facts
  (edit the source item instead, Section 7.3).
- unified_chat_service: [PROMOTE] marker parser (JSON-block per Section
  8.1 spec drift note), stable-UUID assignment for pending_task_lane
  questions/actions preserved by exact text/label match across turns.
- ASSISTANT_SYSTEM_PROMPT: documents [PROMOTE] format, when to/not to
  emit, hallucination guardrails, source_ref handling.
- 17 tests covering parser, stable IDs, service validation, CRUD,
  editability rule, both promote modes, 422 null-synthesis path,
  state_version invariant.

Frontend:
- src/components/pilot/sections/{WhatWeKnow,WhatWeKnowItem,AddNoteButton}
  — green-gradient section above Questions, dashed-circle check, inline
  edit/delete gated by the server's editable flag.
- TaskLane gains a whatWeKnowSlot prop (existing assistant/ folder kept
  per the doc's "rename is opportunistic" guidance).
- AssistantChatPage fetches facts on selectChat and refetches after each
  chat send (so [PROMOTE]-synthesized facts appear immediately); auto-
  opens the lane when facts exist.

Verification: end-to-end smoke against the local docker stack confirms
all five endpoints (list/create/patch/delete/promote) plus the 403
editability rule. pytest suite verifies the same with mocked LLM. Live
[PROMOTE] flow remains untested until used in the UI — the marker shape
is covered by parser tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/router.py

@@ -41,6 +41,7 @@ from app.api.endpoints import (
     scripts,
     script_builder,
     session_branches,
+    session_facts,
     session_handoffs,
     session_resolutions,
     sessions,
@@ -135,6 +136,9 @@ api_router.include_router(network_diagrams.router, dependencies=_tenant_deps)
 # session_handoffs queue router must come before ai_sessions to avoid conflict
 api_router.include_router(session_handoffs.queue_router, dependencies=_tenant_deps)
 api_router.include_router(session_resolutions.router, dependencies=_tenant_deps)
+# session_facts mounts under /ai-sessions/{id}/facts — register before ai_sessions
+# so the {session_id}/facts subpaths take precedence over any future generic catchalls.
+api_router.include_router(session_facts.router, dependencies=_tenant_deps)
 api_router.include_router(ai_sessions.router, dependencies=_tenant_deps)
 api_router.include_router(flow_proposals.router, dependencies=_tenant_deps)
 api_router.include_router(flowpilot_analytics.router, dependencies=_tenant_deps)