scratch matrix approval notifier, add chat-native MCP approval tools

Removes the entire Matrix-based notifier (internal/notifier/) that polled
for pending approvals, sent Matrix alerts, and checked for reaction-based
approve/deny. Approval decisions now work on any chat platform (Hermes
desktop, Telegram, Discord, WhatsApp, CLI) via two new MCP tools:

- list_approvals — query pending/recent approvals by status or entity
- decide_approval — approve/deny via same API endpoint as UI + nomos

Config fields removed: MatrixHomeserver, MatrixUserID, MatrixToken,
MatrixRoomID, ApprovalHMACSecret. Docker notifier: service removed.
Approval HMAC token generation removed (unused by code).

The existing chat-assent path in nomos (cmd/nomos/assent.go) and the
control-room Approve button keep working unchanged — both call the
shared POST /api/v1/approvals/{id}/decision endpoint.

docker-compose.yml

@@ -157,44 +157,6 @@ services:
       retries: 3
       start_period: 90s
 
-  # Notifier (Phase 3) — Matrix alerts
-  notifier:
-    image: oikos-notifier:${OIKOS_VERSION:-latest}
-    build:
-      context: .
-      dockerfile: compose/oikos/Dockerfile
-    restart: unless-stopped
-    profiles: ["dev", "full"]
-    depends_on:
-      seed:
-        condition: service_completed_successfully
-    environment:
-      OIKOS_DATABASE_URL: postgres://oikos:${OIKOS_DB_PASSWORD:-oikos_dev}@postgres:5432/oikos?sslmode=disable
-      OIKOS_DEBUG: "true"
-      OIKOS_APPROVAL_HMAC_SECRET: ${OIKOS_APPROVAL_HMAC_SECRET:-dev-secret}
-      OIKOS_MATRIX_HOMESERVER: ${OIKOS_MATRIX_HOMESERVER:-https://matrix.hubris.network}
-      OIKOS_MATRIX_USER: ${OIKOS_MATRIX_USER:-@hermes:hubris.network}
-      OIKOS_MATRIX_TOKEN: ${OIKOS_MATRIX_TOKEN}
-      OIKOS_MATRIX_ROOM: ${OIKOS_MATRIX_ROOM:-!alerts:hubris.network}
-      # Liveness probe (plan D5): bumps each approval/reaction tick.
-      OIKOS_HEALTH_LISTEN: ":8094"
-      OIKOS_INFISICAL_SITE_URL: ${OIKOS_INFISICAL_SITE_URL:-}
-      OIKOS_INFISICAL_CLIENT_ID: ${OIKOS_INFISICAL_CLIENT_ID:-}
-      OIKOS_INFISICAL_CLIENT_SECRET: ${OIKOS_INFISICAL_CLIENT_SECRET:-}
-      OIKOS_INFISICAL_PROJECT_ID: ${OIKOS_INFISICAL_PROJECT_ID:-}
-      OIKOS_INFISICAL_ENV: ${OIKOS_INFISICAL_ENV:-dev}
-    command: ["notifier"]
-    stop_signal: SIGTERM
-    stop_grace_period: 30s
-    mem_limit: 128m
-    cpus: 0.5
-    healthcheck:
-      test: ["CMD", "wget", "-q", "-O", "-", "http://127.0.0.1:8094/healthz"]
-      interval: 30s
-      timeout: 5s
-      retries: 3
-      start_period: 120s
-
   # Execution worker (Phase 6) — Postgres-backed job queue
   execution-worker:
     image: oikos-execution-worker:${OIKOS_VERSION:-latest}