refactor(host_agent): extract metric-query helpers to a model-free module

The previous commit's integration test failed at collection-time import:
importing plugins.host_agent.routes (which imports the host_agent ORM models at
top level) double-registers host_agent_registrations against the app-loaded
plugin's metadata ("Table already defined").

Move the two pure read helpers (_latest_metrics_for_host, _history_for_host)
plus SOURCE_MODULE / HISTORY_METRICS into plugins/host_agent/metrics_query.py,
which imports only the core PluginMetric model — no plugin models. routes.py
imports them back. The integration test now imports from metrics_query and no
longer trips the loader's registration guard. No behavior change.

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

tests/integration/test_host_metrics.py

@@ -34,7 +34,7 @@ def app():
 def test_latest_distinct_on_and_sql_bucketed_history(app):
     from sqlalchemy import text
     from steward.models.metrics import PluginMetric
-    from plugins.host_agent.routes import (
+    from plugins.host_agent.metrics_query import (
         SOURCE_MODULE, _history_for_host, _latest_metrics_for_host,
     )