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/config.py

@@ -1664,7 +1664,10 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
     # Registry-driven enable for plugin platforms.  Built-ins have explicit
     # blocks above; plugins expose check_fn() which is the single source of
     # truth for "are my env vars set?".  When it returns True, ensure the
-    # platform is enabled so start() will create its adapter.
+    # platform is enabled so start() will create its adapter.  Plugins that
+    # need to seed ``PlatformConfig.extra`` from env vars (e.g. Google Chat's
+    # project_id / subscription_name) can supply ``env_enablement_fn`` on
+    # their PlatformEntry — called here BEFORE adapter construction.
     try:
         from hermes_cli.plugins import discover_plugins
         discover_plugins()  # idempotent
@@ -1680,5 +1683,31 @@ def _apply_env_overrides(config: GatewayConfig) -> None:
             if platform not in config.platforms:
                 config.platforms[platform] = PlatformConfig()
             config.platforms[platform].enabled = True
+            # Seed extras from env if the plugin opted in.
+            if entry.env_enablement_fn is not None:
+                try:
+                    seed = entry.env_enablement_fn()
+                except Exception as e:
+                    logger.debug(
+                        "env_enablement_fn for %s raised: %s", entry.name, e
+                    )
+                    seed = None
+                if isinstance(seed, dict) and seed:
+                    # Extract the home_channel dict (if provided) so we wire it
+                    # up as a proper HomeChannel dataclass.  Everything else is
+                    # merged into ``extra``.
+                    home = seed.pop("home_channel", None)
+                    config.platforms[platform].extra.update(seed)
+                    if isinstance(home, dict) and home.get("chat_id"):
+                        config.platforms[platform].home_channel = HomeChannel(
+                            platform=platform,
+                            chat_id=str(home["chat_id"]),
+                            name=str(home.get("name") or "Home"),
+                            thread_id=(
+                                str(home["thread_id"])
+                                if home.get("thread_id")
+                                else None
+                            ),
+                        )
     except Exception as e:
         logger.debug("Plugin platform enable pass failed: %s", e)