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.

tests/tools/test_send_message_missing_platforms.py

@@ -7,7 +7,6 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 from tools.send_message_tool import (
     _send_dingtalk,
-    _send_homeassistant,
     _send_matrix,
 )
 
@@ -28,6 +27,22 @@ async def _send_mattermost(token, extra, chat_id, message):
     return await _mattermost_standalone_send(pconfig, chat_id, message)
 
 
+# ``_send_homeassistant`` moved into the homeassistant plugin
+# (``plugins/platforms/homeassistant/adapter.py::_standalone_send``).  Same
+# shim pattern as ``_send_mattermost`` above.
+from plugins.platforms.homeassistant.adapter import (
+    _standalone_send as _homeassistant_standalone_send,
+)
+
+
+async def _send_homeassistant(token, extra, chat_id, message):
+    """Pre-migration ``(token, extra, chat_id, message)`` shim around the
+    plugin's ``_standalone_send(pconfig, chat_id, message)``.
+    """
+    pconfig = SimpleNamespace(token=token, extra=extra or {})
+    return await _homeassistant_standalone_send(pconfig, chat_id, message)
+
+
 # ---------------------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------------------