Home Assistant is a bundled plugin now (#40709) and declares
allow_update_command=True on its PlatformEntry. The registry fallback
in _handle_update_command already covers it, so the frozenset entry is
a redundant double-allow — same cleanup #40711 did for Discord and
Mattermost. Adds a registry-fallback test mirroring the existing
discord/mattermost cases.

Move gateway/platforms/slack.py into plugins/platforms/slack/ following the
Discord (#24356) and Home Assistant (#40709) migrations. Advances #41112 /
hardcoded Platform.SLACK touchpoints in core.

  - Adapter file renamed via git mv (history preserved).
  - register() exposes the platform via ctx.register_platform() instead of the
    Platform.SLACK elif in gateway/run.py::_create_adapter().
  - _standalone_send() replaces the legacy _send_slack() helper in
    tools/send_message_tool.py; out-of-process cron delivery (deliver=slack)
    now flows through the registry's standalone_sender_fn. mrkdwn formatting
    moved into the plugin (was applied in _send_to_platform before chunking).
  - _apply_yaml_config() owns the config.yaml slack: -> SLACK_* env bridge
    (require_mention, strict_mention, allow_bots, free_response_channels,
    reactions, allowed_channels), replacing the hardcoded block in
    gateway/config.py.
  - interactive_setup() replaces hermes_cli/setup.py::_setup_slack +
    _write_slack_manifest_and_instruct and the static _PLATFORMS["slack"] dict
    in hermes_cli/gateway.py; setup metadata is discovered dynamically.
  - is_connected() probes SLACK_BOT_TOKEN via hermes_cli.gateway.get_env_value.
  - max_message_length=39000 on the PlatformEntry; the registry fallback in
    send_message_tool covers it (dropped the _MAX_LENGTHS entry).

The SLACK_BOT_TOKEN/SLACK_HOME_CHANNEL env->PlatformConfig seeding and the
_is_user_authorized allowlist maps stay in core (same as Discord/HA/Mattermost).

Bug fixed during migration: the registry-driven plugin-enable pass in
_apply_env_overrides re-enabled any plugin platform whose is_connected()
passed, ignoring an explicit enabled: false. Slack is the first plugin with an
enabled-false-wins test, so it exposed this latent bug (Discord had no such
test). Added an explicit-disable guard (_enabled_explicit + enabled=False ->
skip) and changed the slack env-block to read the flag instead of popping it so
the guard can see it; the flag is still cleared in the final per-platform
cleanup. Restores test_explicit_{top_level,platforms}_slack_enabled_false_wins.

Test imports rewritten across 11 files (gateway.platforms.slack ->
plugins.platforms.slack.adapter). The _setup_slack home-channel tests moved to
tests/gateway/test_slack_plugin_setup.py exercising interactive_setup. The
test_send_message_tool slack-formatting tests now patch the registry
standalone_sender_fn (via _patch_slack_standalone_sender) and assert the
mrkdwn-formatted text reaches the wire.

Validation: 706 targeted tests pass (slack/config/setup/registry/send/media
suites); 18/18 live E2E checks pass (real plugin discovery + registry resolves
SlackAdapter, env-only enable, standalone sender wired, YAML bridge, dynamic
setup discovery).

Move gateway/platforms/slack.py into plugins/platforms/slack/ following the
Discord (#24356) and Home Assistant (#40709) migrations. Advances #41112 /
hardcoded Platform.SLACK touchpoints in core.

  - Adapter file renamed via git mv (history preserved).
  - register() exposes the platform via ctx.register_platform() instead of the
    Platform.SLACK elif in gateway/run.py::_create_adapter().
  - _standalone_send() replaces the legacy _send_slack() helper in
    tools/send_message_tool.py; out-of-process cron delivery (deliver=slack)
    now flows through the registry's standalone_sender_fn. mrkdwn formatting
    moved into the plugin (was applied in _send_to_platform before chunking).
  - _apply_yaml_config() owns the config.yaml slack: -> SLACK_* env bridge
    (require_mention, strict_mention, allow_bots, free_response_channels,
    reactions, allowed_channels), replacing the hardcoded block in
    gateway/config.py.
  - interactive_setup() replaces hermes_cli/setup.py::_setup_slack +
    _write_slack_manifest_and_instruct and the static _PLATFORMS["slack"] dict
    in hermes_cli/gateway.py; setup metadata is discovered dynamically.
  - is_connected() probes SLACK_BOT_TOKEN via hermes_cli.gateway.get_env_value.
  - max_message_length=39000 on the PlatformEntry; the registry fallback in
    send_message_tool covers it (dropped the _MAX_LENGTHS entry).

The SLACK_BOT_TOKEN/SLACK_HOME_CHANNEL env->PlatformConfig seeding and the
_is_user_authorized allowlist maps stay in core (same as Discord/HA/Mattermost).

Bug fixed during migration: the registry-driven plugin-enable pass in
_apply_env_overrides re-enabled any plugin platform whose is_connected()
passed, ignoring an explicit enabled: false. Slack is the first plugin with an
enabled-false-wins test, so it exposed this latent bug (Discord had no such
test). Added an explicit-disable guard (_enabled_explicit + enabled=False ->
skip) and changed the slack env-block to read the flag instead of popping it so
the guard can see it; the flag is still cleared in the final per-platform
cleanup. Restores test_explicit_{top_level,platforms}_slack_enabled_false_wins.

Test imports rewritten across 11 files (gateway.platforms.slack ->
plugins.platforms.slack.adapter). The _setup_slack home-channel tests moved to
tests/gateway/test_slack_plugin_setup.py exercising interactive_setup. The
test_send_message_tool slack-formatting tests now patch the registry
standalone_sender_fn (via _patch_slack_standalone_sender) and assert the
mrkdwn-formatted text reaches the wire.

Validation: 706 targeted tests pass (slack/config/setup/registry/send/media
suites); 18/18 live E2E checks pass (real plugin discovery + registry resolves
SlackAdapter, env-only enable, standalone sender wired, YAML bridge, dynamic
setup discovery).

…ch#40736)

Home Assistant is a bundled plugin now (NousResearch#40709) and declares
allow_update_command=True on its PlatformEntry. The registry fallback
in _handle_update_command already covers it, so the frozenset entry is
a redundant double-allow — same cleanup NousResearch#40711 did for Discord and
Mattermost. Adds a registry-fallback test mirroring the existing
discord/mattermost cases.