feat(docker): per-host collection via the host agent; drop central scrape

Docker collection moves off the central single-socket scrape onto the host
agent, giving Docker a real per-host dimension. The Steward host now reports
its own containers like any other host, and same-named containers on different
hosts no longer collide.

- agent: stdlib UDS Docker client (AF_UNIX HTTP/1.1, Connection: close,
  chunked-aware), collect_docker() ports the cpu%/mem math; sample["docker"]
  added best-effort (silent-skip on absent/unreadable socket). AGENT_VERSION
  1.2.0 → 1.3.0; optional docker_socket config key.
- ingest: host_agent ingest hands per-host container snapshots to the docker
  plugin via a new "docker.persist_host_samples" capability (no hard import,
  no-op when docker disabled), inside a SAVEPOINT so a docker failure never
  sinks the host metrics. Resource names are host-scoped ("<host>/<name>").
- schema: docker_containers re-keyed (host_id, name); docker_metrics gains
  host_id; docker_002 migration DROP+recreates (dev-only, rule 122).
- ui: Docker page + widgets grouped by host with host links; new per-host
  Docker panel embedded on the Hosts hub (gated on docker enabled via a new
  enabled_plugins template context). Replaces the SQLite-only strftime
  bucketing with DB-agnostic Python bucketing.
- provisioning: install/provision playbooks add steward-agent to the docker
  group (best-effort) so the agent can read the socket.
- removed central scrape: docker scheduler.py + scraper.py deleted; plugin.yaml
  socket_path/scrape_interval_seconds/include_stopped dropped (plugin 2.0.0).
- tests: agent docker collector units (math, chunked decode, silent-skip,
  sample shape, config) + integration (host-scoped schema + persistence).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

plugins/docker/__init__.py

@@ -13,10 +13,26 @@ def setup(app: "Quart") -> None:
     _app = app
     from .models import DockerContainer, DockerMetric  # noqa: F401 — register with Base
 
+    # Publish the per-host persist hook so the host agent can hand off the
+    # `docker` array from each sample without importing our models (opportunistic
+    # synergy — host_agent gates on has_capability and no-ops if we're disabled).
+    # required_role=viewer: this is a trusted server-side data-plane write driven
+    # by the authenticated agent ingest, not a user-facing privileged action.
+    from steward.core.capabilities import register_capability
+    from steward.models.users import UserRole
+    from .ingest import persist_host_docker
+    register_capability(
+        "docker.persist_host_samples", persist_host_docker,
+        label="Persist host Docker samples",
+        description="Store per-host container state + metrics pushed by the host agent.",
+        required_role=UserRole.viewer,
+    )
+
 
 def get_scheduled_tasks() -> list:
-    from .scheduler import make_task
-    return [make_task(_app)]
+    # Collection is now agent-driven (pushed to the host_agent ingest); the
+    # central socket scrape was removed. No periodic task to register.
+    return []
 
 
 def get_blueprint():