feat: add host_agent plugin — push-model host resource monitoring

Lightweight stdlib-only Python agent runs on each monitored host,
collects CPU / memory / disk / load / uptime from /proc every 30s,
and POSTs signed payloads to the Roundtable ingest endpoint. One-line
curl-pipe installer creates a hardened systemd unit; admin UI manages
host registrations with rotate / revoke.

- agent.py: 370 LoC single-file daemon, ring buffer + exponential backoff
- ingest route: bearer-token auth, metric expansion into plugin_metrics
- install.sh.j2: systemd unit with NoNewPrivileges / ProtectSystem / ProtectHome
- settings UI: add host / rotate token / delete registration (admin-only)
- dashboard widgets: fleet-glance table + per-host history chart
- stale-agent scheduler: 60s log warning for agents past 180s silence

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

host_agent/migrations/env.py

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+# plugins/host_agent/migrations/env.py
+"""Alembic env.py for the host_agent plugin."""
+import asyncio
+import os
+import sys
+from pathlib import Path
+from logging.config import fileConfig
+
+from sqlalchemy import pool
+from sqlalchemy.engine import Connection
+from sqlalchemy.ext.asyncio import async_engine_from_config
+from alembic import context
+
+sys.path.insert(0, str(Path(__file__).parent.parent.parent.parent))
+
+from roundtable.models.base import Base
+import roundtable.models  # noqa: F401
+from plugins.host_agent.models import HostAgentRegistration  # noqa: F401
+
+config = context.config
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+target_metadata = Base.metadata
+
+
+def _get_url() -> str:
+    import yaml
+    cfg_path = os.environ.get("ROUNDTABLE_CONFIG", "config.yaml")
+    try:
+        with open(cfg_path) as f:
+            cfg = yaml.safe_load(f) or {}
+        url = cfg.get("database", {}).get("url", "")
+    except FileNotFoundError:
+        url = ""
+    return os.environ.get("ROUNDTABLE_DATABASE__URL", url)
+
+
+def run_migrations_offline() -> None:
+    context.configure(
+        url=_get_url(), target_metadata=target_metadata,
+        literal_binds=True, dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def do_run_migrations(connection: Connection) -> None:
+    context.configure(connection=connection, target_metadata=target_metadata)
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_async_migrations() -> None:
+    cfg = config.get_section(config.config_ini_section, {})
+    cfg["sqlalchemy.url"] = _get_url()
+    connectable = async_engine_from_config(cfg, prefix="sqlalchemy.", poolclass=pool.NullPool)
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+    await connectable.dispose()
+
+
+def run_migrations_online() -> None:
+    asyncio.run(run_async_migrations())
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()

host_agent/migrations/versions/host_agent_001_initial.py

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+# plugins/host_agent/migrations/versions/host_agent_001_initial.py
+"""host_agent plugin initial tables
+
+Revision ID: host_agent_001_initial
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "host_agent_001_initial"
+down_revision: Union[str, None] = None
+branch_labels: Union[str, Sequence[str], None] = "host_agent"
+depends_on: Union[str, Sequence[str], None] = ("0004_app_settings",)
+
+
+def upgrade() -> None:
+    op.create_table(
+        "host_agent_registrations",
+        sa.Column("id", sa.String(36), primary_key=True),
+        sa.Column("host_id", sa.String(36),
+                  sa.ForeignKey("hosts.id", ondelete="CASCADE"),
+                  nullable=False, unique=True),
+        sa.Column("token_hash", sa.String(64), nullable=False, unique=True, index=True),
+        sa.Column("token_created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("agent_version", sa.String(32), nullable=True),
+        sa.Column("kernel", sa.String(128), nullable=True),
+        sa.Column("distro", sa.String(128), nullable=True),
+        sa.Column("arch", sa.String(32), nullable=True),
+        sa.Column("last_seen_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), nullable=False),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("host_agent_registrations")