feat(contracts): add infrastructure-maintenance contract

New responsibility contract consolidating host-level system maintenance
and Docker image lifecycle management, filling the gap left by
infrastructure-update (which owns cluster-wide apt waves).

Scope:
- OS package updates on primary host with pre-update snapshot/backup check
- Docker image pulls for LiteLLM, SearXNG, and other running containers
- Container restarts with per-stack health verification
- Post-update verification: LiteLLM, SearXNG, Zulip, Gitea, PM2, Hermes gateways
- Rollback on failure (image/apt/config restore) with circuit breaker

Owner: ops (firstmate secondmate). Trigger: weekly Sunday 2am ET.
Escalation: warning/critical->abiba+mumuni, fatal->abiba+mumuni+kwame.
circuit_breaker: max_retries 2, window 7200, trip_action escalate_to_fatal.
depends_on: infrastructure-monitoring (pre-update health baseline).

Registry:
- Add infrastructure-maintenance to by_category.maintenance, by_domain.infrastructure,
  by_owner.ops (new), by_trigger.scheduled, by_sensitivity.high
- Add 'ops' to owners list
- Move infrastructure-update owner abiba -> ops (in contracts entry + by_owner index)

Also adds '## Maintaining this file' section to AGENTS.md per fm-ensure-agents-md.

AGENTS.md

@@ -128,3 +128,10 @@ safe-mutate --verify "CMD" [--expect "PATTERN"] --mutate "CMD" [--reason "WHY"]
 Read the [Authoring Guide](docs/AUTHORING-GUIDE.md) before writing any new contract.
 It covers the full process: verify → draft → lint → review → ship, with templates
 and style rules.
+
+## Maintaining this file
+
+Keep this file for knowledge useful to almost every future agent session in this project.
+Do not repeat what the codebase already shows; point to the authoritative file or command instead.
+Prefer rewriting or pruning existing entries over appending new ones.
+When updating this file, preserve this bar for all agents and keep entries concise.