feat(mcp): optional bearer-token auth via SUPERVISOR_MCP_TOKEN

Enables exposing the supervisor MCP via Tailscale Funnel for claude.ai
custom-connector tests. Auth is opt-in: empty SUPERVISOR_MCP_TOKEN
preserves the existing unauthenticated behavior for tailnet-internal
callers and local dev.

When the token is set, every request must carry
"Authorization: Bearer <token>" or it is rejected with HTTP 401 and a
JSON-RPC -32001 error. Comparison uses crypto/subtle.ConstantTimeCompare;
the token value and the supplied header are never logged.

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

README.md

@@ -111,6 +111,7 @@ The supervisor probes connectivity at call time:
 | `SUPERVISOR_SESSIONS_DIR` | `./brain/sessions` | JSONL session logs |
 | `INGEST_BASE_URL` | `http://localhost:3300` | Supervisor → ingestion |
 | `LITELLM_BASE_URL` | — | LiteLLM proxy for Tier 2 model routing |
+| `SUPERVISOR_MCP_TOKEN` | — | Optional bearer token for the supervisor MCP HTTP endpoint; when empty, no auth is enforced |
 
 ## Phase 2 (planned)