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.

AGENTS.md

@@ -56,7 +56,7 @@ Endpoint: `https://mcp.hubris.network/mcp`. Every call needs
 enrollment and `/healthz` (see "Authentication" below for where the token
 comes from).
 
-Available tools (65 total — the authoritative list; do not hardcode the count
+Available tools (67 total — the authoritative list; do not hardcode the count
 elsewhere; regenerate from `internal/mcp/` when tools change):
 
   💡 Slug param alias: all entity-lookup tools now accept `slug` in addition to
@@ -103,6 +103,8 @@ elsewhere; regenerate from `internal/mcp/` when tools change):
     list_executions(entity_slug, status, limit=25) — cursor-paginated execution history
     list_entity_sessions(entity_slug) — active Nomos sessions linked to an entity
     get_dashboard_summary() — fleet overview: counts, health, signals, approvals
+    list_approvals(status, entity_slug, limit) — list pending/recent approvals; filter by status (pending, approved, denied) or entity
+    decide_approval(approval_id, decision) — approve or deny a pending execution; calls the same API endpoint as the Approve button in the UI
     get_secret(key, path, environment) — retrieve a secret from the Infisical vault
     list_secrets(path_prefix) — list secret keys in the Infisical vault
     set_secret(key, value, path, environment) — store/update a secret (requires approval)
@@ -187,13 +189,14 @@ The DB is the truth. The old wiki files are archived at `archive/knowledge/`
 - **Actions** (restart, logs, apt, pct exec, or anything else): Nomos calls
   `run` (the general execution primitive) via MCP. `reversible_low`/read-only actions execute
   immediately; `config_mutation` and `destructive` actions are queued for
-  operator approval via Matrix or the control-room UI's Operations page.
+  operator approval via the App button in the control-room UI or via
+  `list_approvals`/`decide_approval` MCP tools from any agent.
 - **Secrets**: managed by Infisical (`oikos secret` subcommand for
   migration). Never hardcode secrets — use env vars from `.env`.
 - **Mutations** (restart, edit configs, etc.): classified against
   `seeds/policy.yaml`. `reversible_low` actions auto-execute;
   `config_mutation`/`destructive` actions require approval — granted by
-  the operator via Matrix reply or the control-room UI, not a CLI flag.
+  the operator via the App button or `decide_approval` MCP tool call.
   See OIKOS.md.
 
 ## 7. Communication mode