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/vault.yml.example

@@ -22,7 +22,11 @@ vault_mqtt_c2_pass: "CHANGE_ME"
 vault_mqtt_dynsec_admin_pass: "CHANGE_ME"  # openssl rand -hex 32 — must be >=12 chars, plugin-enforced minimum
 
 # ── C2 Core ───────────────────────────────────────────────────────────────────
-vault_service_key: ""            # openssl rand -hex 32
+vault_service_key: ""            # openssl rand -hex 32 — the Discord bot's key
+# The work-session agent's own key for GET/PUT /admin/features. Generate a
+# SEPARATE value — never a copy of vault_service_key, or a flag flip cannot be
+# attributed to the agent vs the bot (server-26#64).
+vault_agent_service_key: ""      # openssl rand -hex 32
 vault_enrollment_token: ""       # openssl rand -hex 32 — fleet-wide, shared by every node's POST /nodes/enroll
 vault_openai_api_key: ""
 vault_google_maps_api_key: ""