refactor(gateway): migrate Home Assistant adapter to bundled plugin

Move gateway/platforms/homeassistant.py into plugins/platforms/homeassistant/
following the same shape as the Mattermost and Discord migrations.

  - Adapter file is renamed via git mv (history is preserved).
  - register() exposes the platform via the plugin system instead of the
    hardcoded Platform.HOMEASSISTANT elif in gateway/run.py::build_adapter().
  - _standalone_send() replaces the legacy _send_homeassistant() helper in
    tools/send_message_tool.py.  Out-of-process cron delivery
    (deliver=homeassistant from a cron process not co-located with the
    gateway) now flows through the registry's standalone_sender_fn path
    instead of the hardcoded elif.
  - _is_connected() probes HASS_TOKEN via hermes_cli.gateway.get_env_value
    so existing connected-platform checks behave identically.

The HASS_TOKEN / HASS_URL env-to-PlatformConfig seeding in
gateway/config.py stays in core — same pattern bluebubbles, mattermost,
and discord migrations followed.  No setup_fn or apply_yaml_config_fn is
registered because Home Assistant has no _setup_homeassistant wizard in
hermes_cli/setup.py and no homeassistant: YAML block in config.yaml today;
setup runs through the existing hermes_cli/tools_config.py toolset wizard.

Test imports were rewritten across tests/gateway/test_homeassistant.py,
tests/integration/test_ha_integration.py, and
tests/tools/test_send_message_missing_platforms.py; the legacy
(token, extra, chat_id, message)-shaped _send_homeassistant call site is
preserved via a small SimpleNamespace shim in
test_send_message_missing_platforms.py (same approach used when
mattermost moved).

  - Focused HA suites (64 tests across the three rewritten files) pass.
  - Broader gateway/cron sweep produces 10 failures identical to main
    baseline (telegram approval/model-picker xdist isolation flakes,
    wecom_callback defusedxml issue, cron script_timeout fixture issue).
    Zero net new failures.

tools/send_message_tool.py

@@ -788,8 +788,6 @@ async def _send_to_platform(platform, pconfig, chat_id, message, thread_id=None,
             result = await _send_sms(pconfig.api_key, chat_id, chunk)
         elif platform == Platform.MATRIX:
             result = await _send_matrix(pconfig.token, pconfig.extra, chat_id, chunk)
-        elif platform == Platform.HOMEASSISTANT:
-            result = await _send_homeassistant(pconfig.token, pconfig.extra, chat_id, chunk)
         elif platform == Platform.DINGTALK:
             result = await _send_dingtalk(pconfig.extra, chat_id, chunk)
         elif platform == Platform.FEISHU:
@@ -1486,29 +1484,6 @@ async def _send_matrix_via_adapter(pconfig, chat_id, message, media_files=None,
             pass
 
 
-async def _send_homeassistant(token, extra, chat_id, message):
-    """Send via Home Assistant notify service."""
-    try:
-        import aiohttp
-    except ImportError:
-        return {"error": "aiohttp not installed. Run: pip install aiohttp"}
-    try:
-        hass_url = (extra.get("url") or os.getenv("HASS_URL", "")).rstrip("/")
-        token = token or os.getenv("HASS_TOKEN", "")
-        if not hass_url or not token:
-            return {"error": "Home Assistant not configured (HASS_URL, HASS_TOKEN required)"}
-        url = f"{hass_url}/api/services/notify/notify"
-        headers = {"Authorization": f"Bearer {token}", "Content-Type": "application/json"}
-        async with aiohttp.ClientSession(timeout=aiohttp.ClientTimeout(total=30)) as session:
-            async with session.post(url, headers=headers, json={"message": message, "target": chat_id}) as resp:
-                if resp.status not in {200, 201}:
-                    body = await resp.text()
-                    return _error(f"Home Assistant API error ({resp.status}): {body}")
-        return {"success": True, "platform": "homeassistant", "chat_id": chat_id}
-    except Exception as e:
-        return _error(f"Home Assistant send failed: {e}")
-
-
 async def _send_dingtalk(extra, chat_id, message):
     """Send via DingTalk robot webhook.