feat(gateway): generic plugin hooks for env enablement + cron delivery

Widen the platform-plugin surface so plugins can self-configure from env
vars and opt into cron home-channel delivery without editing core files.
Closes the scope gap that forced every new platform (Google Chat, Teams,
IRC, future) to either touch gateway/config.py, cron/scheduler.py, and
hermes_cli/config.py or live without env-only setup.

Changes:

- gateway/platform_registry.py: two new optional PlatformEntry fields.
  - env_enablement_fn: () -> Optional[dict]. Called during
    _apply_env_overrides BEFORE the adapter is constructed. Returned
    dict fields are merged into PlatformConfig.extra; the special
    'home_channel' key (if present) becomes a proper HomeChannel
    dataclass on the PlatformConfig.
  - cron_deliver_env_var: name of the *_HOME_CHANNEL env var. When set,
    the plugin platform is a valid cron deliver= target and cron reads
    the env var to resolve the default chat/room ID.

- gateway/config.py: the existing plugin-platform enable pass at the
  bottom of _apply_env_overrides now calls env_enablement_fn and seeds
  extras/home_channel. No effect on plugins that don't set the new
  field.

- cron/scheduler.py: _is_known_delivery_platform and
  _resolve_home_env_var fall through to the registry when the platform
  isn't in the hardcoded built-in sets. New _iter_home_target_platforms
  helper iterates built-ins + plugin platforms for the deliver=origin
  fallback.

- gateway/run.py: _home_target_env_var now consults the new resolver so
  plugin-defined home channels work for non-cron call sites too.

- hermes_cli/config.py: new _inject_platform_plugin_env_vars() sibling
  of _inject_profile_env_vars(). Scans plugins/platforms/*/plugin.yaml
  at import time and contributes entries to OPTIONAL_ENV_VARS so
  'hermes config' UI discovers them. Supports bare-string and rich-dict
  requires_env entries plus a new optional_env list for non-required
  vars (home channels, allowlists).

All additions are strictly opt-in. Existing plugins (IRC, Teams,
image_gen, memory) see zero behavior change until they adopt the new
fields.

gateway/platform_registry.py

@@ -110,6 +110,21 @@ class PlatformEntry:
     # Do not use markdown.").  Empty string = no hint.
     platform_hint: str = ""
 
+    # ── Env-driven auto-configuration ──
+    # Optional: read env vars, return a dict of ``PlatformConfig.extra`` fields
+    # to seed when the platform is auto-enabled.  Called during
+    # ``_apply_env_overrides`` BEFORE the adapter is constructed, so
+    # ``gateway status`` etc. can reflect env-only configuration without
+    # instantiating the adapter.  Return ``None`` (or an empty dict) to skip.
+    # Signature: () -> Optional[dict[str, Any]]
+    env_enablement_fn: Optional[Callable[[], Optional[dict]]] = None
+
+    # Optional: home-channel env var name for cron/notification delivery
+    # (e.g. ``"IRC_HOME_CHANNEL"``).  When set, ``cron.scheduler`` treats this
+    # platform as a valid ``deliver=<name>`` target and reads the env var to
+    # resolve the default chat/room ID.  Empty = no cron home-channel support.
+    cron_deliver_env_var: str = ""
+
 
 class PlatformRegistry:
     """Central registry of platform adapters.