Close the /admin/features side-door that needed a container shell to flip AI spend

Board minutes #62 Decision 2 (server-26#64), due 2026-08-31. CTO draft #60
finding 1 and CISO draft #61 finding 3 reached this independently.

GET/PUT /admin/features accepted only a Firebase admin token, so the unattended
runbook had no headless path and SSHed into the c2-core container to write
config/ai_features with the admin SDK. Moving a platform-wide AI cost switch
required a full container shell, and set_flags() wrote no audit entry either
way, so a flag flip was unattributable however it happened.

- New agent_service_key (AGENT_SERVICE_KEY), deliberately separate from the
  Discord bot's service_key. Sharing one key would collapse two principals into
  a single unattributable identity in every log line, and the bot has no
  business flipping AI flags regardless.
- require_agent_key_or_admin accepts the agent key or a Firebase admin, and
  rejects the Discord key. The "key is configured" guard is load-bearing:
  compare_digest("", "") is a match, so a deployment that never set the key
  would otherwise accept an empty credential.
- set_flags() writes an audit_log entry with before/after values and the actor,
  wrapped so an audit failure cannot lose the flag write or 500 the route.
- Cascade helper sets the global doc and every system carrying an ai_flags
  override in one call. A global False already beats everything, but a system
  False beats a global True, so turning AI *on* could half-apply and leave a
  radio system hot after shutoff. It scans for the override rather than
  hardcoding the two known system IDs, so a new system cannot silently defeat
  it.
- cascade defaults to False. PUT /systems/{id}/ai-flags and the AiFlagsPanel
  toggle mean a per-system override is deliberate operator intent; cascading by
  default would erase it on any unrelated global flip. The runbook opts in.

Issue items 5 and 6 (retiring the SSH path from drb-worksession.md) are NOT
done here and the runbook is untouched. The credential does not exist in
production yet, so the SSH path is still the only one that works; retiring it
now would break the next unattended run. Owner activation is recorded on #64.

Tests 273 -> 289.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

infra/ansible/roles/deploy/templates/c2-core.env.j2

@@ -30,6 +30,14 @@ GEMINI_API_KEY={{ vault_gemini_api_key }}
 SERVICE_KEY={{ vault_service_key }}
 ENROLLMENT_TOKEN={{ vault_enrollment_token }}
 
+# Agent/automation key for the unattended work session's headless routes
+# (GET/PUT /admin/features). MUST NOT equal vault_service_key: that one is the
+# Discord bot's, and one shared value would make the bot and the agent the same
+# unattributable principal in audit_log (server-26#64). default('') so a vault
+# that predates this key still templates instead of failing the play; blank
+# just leaves the agent path closed.
+AGENT_SERVICE_KEY={{ vault_agent_service_key | default('') }}
+
 # Bare domain, not app.<domain>: the frontend is served on {{ domain }} itself
 # (see Caddyfile.j2 — only api. and the bare name have DNS records). This said
 # app.{{ domain }} while the browser origin was https://{{ domain }}, so every