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/core/settings.py

@@ -79,6 +79,10 @@ DEFAULTS: dict[str, Any] = {
     "thresholds.load_warn": 80, "thresholds.load_crit": 100,
     "thresholds.temp_warn": 70, "thresholds.temp_crit": 85,
     "thresholds.uptime_warn": 99.0, "thresholds.uptime_crit": 95.0,
+    # Days left on an HTTPS monitor's certificate (lower is worse), and how
+    # many container deaths within an hour count as crash-looping.
+    "thresholds.tls_warn_days": 14, "thresholds.tls_crit_days": 3,
+    "thresholds.crash_loop_deaths": 3,
     # Docker time-series retention (rule 25 — tunable, no restart). Raw 30s
     # samples are heavy, so keep a short raw window then roll up to hourly
     # averages kept much longer; lifecycle events are light, keep a month.
@@ -379,6 +383,8 @@ def to_thresholds_cfg(settings: dict[str, Any]) -> dict:
         "temp":    {"warn": g("thresholds.temp_warn", 70), "crit": g("thresholds.temp_crit", 85), "dir": "high"},
         "latency": {"warn": g("ping.threshold.good_ms", 50), "crit": g("ping.threshold.warn_ms", 200), "dir": "high"},
         "uptime":  {"warn": g("thresholds.uptime_warn", 99.0), "crit": g("thresholds.uptime_crit", 95.0), "dir": "low"},
+        "tls_days": {"warn": g("thresholds.tls_warn_days", 14), "crit": g("thresholds.tls_crit_days", 3), "dir": "low"},
+        "crash_loop": {"warn": g("thresholds.crash_loop_deaths", 3), "crit": g("thresholds.crash_loop_deaths", 3), "dir": "high"},
     }