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.

CLIENTS.md

@@ -23,7 +23,7 @@ Docker stack on mac-mini and exposes an MCP server + REST API.
 | Change history | MCP `get_change_history` |
 | State snapshot (health, disk, drift) | MCP `get_state_snapshot` |
 | Secrets (Infisical) | REST API + `oikos secret` CLI |
-| Approval tokens | Matrix via notifier |
+| Approve/deny pending executions | MCP `list_approvals`, `decide_approval` (chat-native, works on any platform) |
 | Run a command on a host/LXC (policy-gated) | MCP `run` |
 | Record a discovered fact/relationship | MCP `update_entity_attributes`, `create_relationship`, `upsert_knowledge` |