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.

scripts/cutover-checklist.md

@@ -6,7 +6,7 @@ Status: [x] = done, [ ] = pending
 
 - [x] **Backup**: `pg_dump oikos > backups/pre-cutover-20260707.sql` (145K)
 - [x] **CI green**: pushed to main, `.gitea/workflows/ci.yml` exists
-- [x] **Deploy test**: Docker stack running with api + scheduler + notifier + nomos
+- [x] **Deploy test**: Docker stack running with api + scheduler + nomos
 - [x] **Caddy config**: `compose/caddy/Caddyfile.oikos` pushed to `dtoro/caddy-conf` (ed20908). Auto-deploys to caddy (121).
 - [x] **DNS**: `oikos.hubris.network` already resolves to 192.168.8.175 (mac-mini mesh)
 - [x] **Secrets**: Infisical bootstrapped + migration complete 2026-07-07. All 11 SOPS secrets migrated to Infisical (oikos project, dev env). Machine identity `oikos-api` has RW access verified via Go SDK. ENCRYPTION_KEY must be 32-char raw string (docs incorrect). SOPS fallback preserved for DR. secrets-issuance decommissioned — stopped/disabled on apps/105; superseded by Infisical.