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>

plugins/docker/__init__.py

@@ -49,6 +49,16 @@ def get_blueprint():
     return docker_bp
 
 
+def register_mcp_tools(register) -> None:
+    from .mcp_tools import register_tools
+    register_tools(register)
+
+
+async def mcp_overview(db) -> list[dict]:
+    from .mcp_tools import mcp_overview as findings
+    return await findings(db)
+
+
 def get_nav() -> list[dict]:
     # Surfaces in the sidebar's Infrastructure group (fleet-wide container view).
     return [{"label": "Docker", "href": "/plugins/docker/"}]