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/AddNoteButton.tsx

@@ -0,0 +1,87 @@
+/**
+ * "+ Add a note" affordance for the What-we-know section.
+ *
+ * Inline composer that posts a `user_note` fact when the engineer wants to
+ * record something the AI didn't surface (a hunch, an observation, a piece
+ * of customer context). Per FLOWPILOT-MIGRATION.md Section 3.1.
+ */
+import { useState } from 'react'
+import { Plus, Check } from 'lucide-react'
+
+interface AddNoteButtonProps {
+  onAdd: (text: string, summary: string | null) => Promise<void> | void
+}
+
+export function AddNoteButton({ onAdd }: AddNoteButtonProps) {
+  const [open, setOpen] = useState(false)
+  const [text, setText] = useState('')
+  const [summary, setSummary] = useState('')
+  const [busy, setBusy] = useState(false)
+
+  const reset = () => {
+    setText('')
+    setSummary('')
+    setOpen(false)
+  }
+
+  const handleSubmit = async () => {
+    if (!text.trim()) return
+    setBusy(true)
+    try {
+      await onAdd(text.trim(), summary.trim() || null)
+      reset()
+    } finally {
+      setBusy(false)
+    }
+  }
+
+  if (!open) {
+    return (
+      <button
+        onClick={() => setOpen(true)}
+        className="flex items-center gap-1.5 text-[0.75rem] text-muted-foreground hover:text-accent-text transition-colors pl-1 mt-1"
+      >
+        <Plus size={12} />
+        Add a note
+      </button>
+    )
+  }
+
+  return (
+    <div className="rounded-lg border border-default bg-card p-3 mb-2">
+      <textarea
+        autoFocus
+        value={text}
+        onChange={(e) => setText(e.target.value)}
+        placeholder="What did you observe or confirm?"
+        className="w-full rounded-md border border-default bg-input px-2.5 py-1.5 text-[0.8125rem] text-heading placeholder:text-muted-foreground resize-y min-h-[44px] max-h-[140px] focus:outline-none focus:border-accent focus:ring-1 focus:ring-accent/30"
+        rows={2}
+      />
+      <input
+        type="text"
+        value={summary}
+        onChange={(e) => setSummary(e.target.value)}
+        placeholder="Short label (optional)"
+        className="mt-1.5 w-full rounded-md border border-default bg-input px-2.5 py-1 text-[0.75rem] text-heading placeholder:text-muted-foreground focus:outline-none focus:border-accent focus:ring-1 focus:ring-accent/30"
+      />
+      <div className="mt-1.5 flex items-center gap-2">
+        <button
+          onClick={handleSubmit}
+          disabled={busy || !text.trim()}
+          className="flex items-center gap-1 rounded-md bg-accent px-2.5 py-1 text-[0.75rem] font-medium text-white disabled:opacity-40 hover:bg-accent-hover transition-colors"
+        >
+          <Check size={11} /> Add
+        </button>
+        <button
+          onClick={reset}
+          disabled={busy}
+          className="text-[0.75rem] text-muted-foreground hover:text-heading"
+        >
+          Cancel
+        </button>
+      </div>
+    </div>
+  )
+}
+
+export default AddNoteButton