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>

frontend/src/components/pilot/sections/WhatWeKnow.tsx

@@ -0,0 +1,74 @@
+/**
+ * What-we-know section — the load-bearing structural feature of the FlowPilot
+ * task lane (per FLOWPILOT-MIGRATION.md Section 0 architectural claim #2).
+ *
+ * Owns the list of confirmed facts for a session. Facts arrive via three paths:
+ *   1. AI [PROMOTE] markers parsed in unified_chat_service (most common)
+ *   2. Engineer "+ Add a note" (manual user_note)
+ *   3. Engineer-initiated promotion of a question/check (future affordance)
+ *
+ * This component is the parent's contract: it takes a fact list + handlers
+ * and renders the section. Loading/refresh logic lives in the parent
+ * (AssistantChatPage) so it can coordinate with the chat send cycle.
+ */
+import { cn } from '@/lib/utils'
+import type { SessionFact } from '@/api/sessionFacts'
+import { WhatWeKnowItem } from './WhatWeKnowItem'
+import { AddNoteButton } from './AddNoteButton'
+
+interface WhatWeKnowProps {
+  facts: SessionFact[]
+  onAddNote: (text: string, summary: string | null) => Promise<void> | void
+  onUpdateFact: (factId: string, text: string, summary: string | null) => Promise<void> | void
+  onDeleteFact: (factId: string) => Promise<void> | void
+  loading?: boolean
+}
+
+export function WhatWeKnow({
+  facts,
+  onAddNote,
+  onUpdateFact,
+  onDeleteFact,
+  loading,
+}: WhatWeKnowProps) {
+  const count = facts.length
+
+  return (
+    <section
+      className={cn(
+        'rounded-lg p-3 -mx-1 mb-1',
+        // Subtle green-to-transparent gradient distinguishes this section
+        // from the rest of the lane (mockup 01-session-primary.png).
+        'bg-gradient-to-b from-success/[0.05] to-transparent',
+      )}
+    >
+      <div className="sticky top-0 z-10 pb-2" style={{ background: 'var(--color-bg-page)' }}>
+        <div className="flex items-center gap-2 text-[10px] font-semibold uppercase tracking-[1.2px] text-muted-foreground pl-0.5">
+          <span className="w-1.5 h-1.5 rounded-full bg-success" />
+          What we know
+          <span className="text-muted-foreground">·</span>
+          <span className="tabular-nums">{count}</span>
+        </div>
+      </div>
+
+      {count === 0 && !loading && (
+        <div className="text-[0.75rem] text-muted-foreground italic px-1 py-2">
+          Nothing confirmed yet — facts appear here as the engineer answers questions and runs checks.
+        </div>
+      )}
+
+      {facts.map((fact) => (
+        <WhatWeKnowItem
+          key={fact.id}
+          fact={fact}
+          onSave={(text, summary) => onUpdateFact(fact.id, text, summary)}
+          onDelete={() => onDeleteFact(fact.id)}
+        />
+      ))}
+
+      <AddNoteButton onAdd={onAddNote} />
+    </section>
+  )
+}
+
+export default WhatWeKnow