Summary

Moves the Home Assistant gateway adapter out of gateway/platforms/homeassistant.py and into plugins/platforms/homeassistant/ as a bundled plugin, following the same shape as the recent Mattermost (#31748), Discord, Teams, and other platform-plugin migrations.

After this PR, every consumer of the HA adapter goes through PluginManager.discover_and_load() → platform_registry.get("homeassistant") instead of a hardcoded Platform.HOMEASSISTANT import branch.

What moves into the plugin

New files:

• plugins/platforms/homeassistant/__init__.py — exposes register
• plugins/platforms/homeassistant/plugin.yaml — manifest declaring HASS_TOKEN (required) and HASS_URL (optional)
• plugins/platforms/homeassistant/adapter.py — renamed adapter with an appended register() block

What stays in core (intentionally)

• The HASS_TOKEN / HASS_URL env-to-PlatformConfig seeding at gateway/config.py:1392-1401. The bluebubbles, mattermost, and discord migrations all kept their equivalent block in gateway/config.py rather than moving it into an env_enablement_fn. Keeping the same pattern here means PR feat(homeassistant): split tool vs platform env vars #29199 (split tool vs platform env vars) collides at most on that small block, not on the renamed file.
• Platform.HOMEASSISTANT enum literal — used as a dict key throughout the codebase; removing it is a separate refactor with no real benefit (per the established migration playbook).
• tools/homeassistant_tool.py — the HA REST/WS toolset that lets the agent call HA services from a prompt. It is a separate concern from this messaging adapter and is not affected.

What does not need a hook

• setup_fn — Home Assistant has no _setup_homeassistant function in hermes_cli/setup.py and is not in hermes_cli/gateway.py::_builtin_setup_fn(). Setup runs through the existing toolset wizard in hermes_cli/tools_config.py, which is unchanged by this PR.
• apply_yaml_config_fn — no homeassistant: block currently exists in config.yaml; there is nothing to translate.
• cron_deliver_env_var — Home Assistant has no notion of a "home channel" the way Discord / Slack / Mattermost do. Cron deliver=homeassistant still works via standalone_sender_fn; it just always requires an explicit chat_id (the HA notify target, e.g. mobile_app_pixel_8).
• allowed_users_env / allow_all_env — the HA adapter consumes the WebSocket event bus, not user-to-user messages; auth is per-token, not per-user.

Why _standalone_send uses notify.notify, not persistent_notification.create

The live adapter's send() calls persistent_notification.create (a notification visible in the HA UI). The legacy _send_homeassistant helper in tools/send_message_tool.py always used notify.notify (which triggers an actual HA notify service, e.g. push to a phone). Out-of-process cron delivery preserves the legacy helper's behavior — phones get pushed, not just an HA dashboard banner. The two surfaces were already different before this PR and this migration does not change that.

Verification

# Plugin discovery picks the migrated platform up
from hermes_cli.plugins import PluginManager
mgr = PluginManager(); mgr.discover_and_load(force=True)
from gateway.platform_registry import platform_registry
sorted(e.name for e in platform_registry.all_entries())
# → ['discord', 'google_chat', 'homeassistant', 'irc', 'line',
#    'mattermost', 'ntfy', 'simplex', 'teams']
e = platform_registry.get('homeassistant')
# e.source == 'plugin', e.standalone_sender_fn is not None,
# e.required_env == ['HASS_TOKEN'], e.emoji == '🏠'

Tests

• tests/gateway/test_homeassistant.py — 41 tests, import paths updated to plugins.platforms.homeassistant.adapter, all mock.patch(...) strings updated. Passes.
• tests/integration/test_ha_integration.py — adapter import path updated. Passes.
• tests/tools/test_send_message_missing_platforms.py — the legacy (token, extra, chat_id, message)-shaped _send_homeassistant is preserved via a small SimpleNamespace shim that wraps the plugin's _standalone_send(pconfig, chat_id, message). This mirrors the shim mattermost added when it migrated, so the existing 4 test bodies don't have to be rewritten. Passes.

Focused suite: 64 passed.

Broader gateway + cron sweep: 6218 passed, 10 failures — identical set to the same sweep run against unmodified main (telegram approval/model-picker xdist isolation flakes, wecom_callback defusedxml issue, cron/test_cron_script::test_script_timeout subprocess-timeout fixture issue). Zero net new failures.

Context for in-flight HA PRs

There are several open HA PRs that touch gateway/platforms/homeassistant.py (#29199, #29388, #31389, #27270, #27272, #24053, #23643). After this migration the file is at plugins/platforms/homeassistant/adapter.py — git rename detection should make rebases largely mechanical, and the env-to-PlatformConfig block they care about stays in gateway/config.py. Happy to help with any rebase.