Brain: inject identity into the live prompt + surface AGENTS.md in the command center

Gap 1 — the brain's identity sections were stored but UI-only. cm-runtime/brain.rs
`compose_system` now folds the brain's AGENTS.md ("## How I operate") + personality
into the live system prompt (after the Postgres-authoritative base, before skills +
memory; falls back to the brain's soul_md when the base is empty). Mirrors the
OpenClaw/ZeroClaw render order.

Mapping — expose `agent_md` on `GET /api/claws/{id}/brain` (ClawBrainResponse) and
render it as a collapsible "HOW I OPERATE · AGENTS.md" card in the command center's
BRAIN column (RawBrain gains agent_md; richBrain passes it through). So the section
that's now in the prompt is also visible in the UI.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

frontend/src/components/dashboard/anatomy-cards.tsx

@@ -13,6 +13,7 @@ export const mono = "'JetBrains Mono', ui-monospace, monospace";
 export type RawBrain = {
   exists: boolean;
   system_prompt: string | null;
+  agent_md: string | null;
   personality: string | null;
   skills: { name: string; body: string }[];
   tools: { name: string; state: string }[];