Add agent memory: linked note store, MCP server, recall hook, and graph UI

The distilled knowledge layer over the raw log/transcript history, inspired by
TencentDB-Agent-Memory's memory-hub model, adapted to handler's invariants
(state lives only in the database; workers stay stateless and disposable):

- memory_notes + memory_links tables (migration 0014) with a portable DAL:
  scoped listing/search (project + global), idempotent linking, and a one-read
  graph. Deleting an agent nulls attribution but keeps its notes; deleting a
  project removes its notes and edges, leaving global knowledge intact.
- /memory API routes: reads on the normal token, note/link authoring on the
  admin token, plus GET /memory/graph for the dashboard.
- Bundled handler-memory MCP server (python -m handler.mcpserver), a
  dependency-free stdio JSON-RPC implementation injected into every launch's
  --mcp-config ahead of the DB connectors and allowlisted in generated
  settings, exposing memory_search / memory_get / memory_save / memory_link.
  Identity and DATABASE_URL arrive via the spawn env, same as hooks.
- SessionStart recall hook: injects the most recent notes in scope as
  additional context at session start, best-effort, never blocking.
- Memory page in the web UI: a hand-rolled force-directed SVG graph of the
  note web (colored by kind, hover highlights, click-through details), plus
  note/link authoring — the app's first visualization, no chart dependency.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WYqkoYPX8NAo1V2KyXr1pk

frontend/app/memory/page.tsx

@@ -0,0 +1,13 @@
+/* Memory page. The shell (sidebar, banners, store) comes from the root layout; this
+ * route contributes only its section, in the shared scroll frame. */
+"use client";
+
+import { MemorySection } from "@/components/sections/MemorySection";
+
+export default function MemoryPage() {
+  return (
+    <div className="main-scroll">
+      <MemorySection />
+    </div>
+  );
+}