feat(mcp): docker and host-agent tools; TLS and crash-loop cutoffs in Settings

MCP-5 of milestone 461 "MCP server: Steward as an LLM-inspectable,
LLM-operable hub". Contributed through each plugin's register_mcp_tools
hook, so they exist only while the plugin is enabled.

docker (viewer, read-only):
- list_containers: filter by host, status, compose project, or problem
  (restarting, crash_looping, unhealthy, oom_killed, exited_nonzero);
  problems sort first and each row names what is wrong.
- get_container: state, CPU/memory history across the raw/hourly seam,
  lifecycle events. A unique part of a swarm task name is enough.
- list_swarm_services: replicas per node (agent-less placements as ghosts)
  and the swarm's nodes.
- search_container_logs: text (% and _ literal), stream, range, context
  lines, paging by a full-precision `before` cursor; over-long lines keep
  both ends and say how much of the middle was cut.
- get_docker_disk_usage: bytes per host, reclaimable first.
- mcp_overview: failing containers and under-replicated swarm services
  join get_overview.

host_agent: list_agents (reporting / stale / pending, version, OS), and an
overview finding for agents registered over 30 minutes ago that never
checked in.

Settings > Thresholds gains the TLS-expiry warn/crit days (were 14/3 in
code) and the crash-loop deaths-per-hour count (3), which get_overview
and the docker tools now read.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/settings/thresholds.html

@@ -40,6 +40,18 @@
   {{ pair("Temperature",  "temp_warn", "thresholds.temp_warn", "temp_crit", "thresholds.temp_crit", "°C", "Hottest reported sensor per host.") }}
   {{ pair("Ping latency", "latency_warn", "ping.threshold.good_ms", "latency_crit", "ping.threshold.warn_ms", "ms", "Round-trip time. Shared with the Ping widget's good/warn colouring.") }}
   {{ pair("Uptime / SLA", "uptime_warn", "thresholds.uptime_warn", "uptime_crit", "thresholds.uptime_crit", "%", "Lower is worse — amber below warn, red below crit.", step="0.1") }}
+  {{ pair("TLS certificate expiry", "tls_warn", "thresholds.tls_warn_days", "tls_crit", "thresholds.tls_crit_days", "days left", "HTTPS monitors whose certificate expires within these many days are flagged. Lower is worse.") }}
+
+  <div class="form-group" style="margin-bottom:0.25rem;">
+    <label>Container crash loop <span style="color:var(--text-muted);font-size:0.8rem;">(deaths per hour)</span></label>
+    <div style="margin-top:0.25rem;">
+      <input type="number" name="crash_loop_deaths" min="1" step="1"
+             value="{{ settings['thresholds.crash_loop_deaths'] }}" style="max-width:110px;">
+    </div>
+    <div style="font-size:0.78rem;color:var(--text-muted);margin-top:0.3rem;">
+      A container that dies (or is OOM-killed) this many times within an hour is reported as crash-looping.
+    </div>
+  </div>
 </div>
 
 {% macro days(label, field, key, hint) %}