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.

cron/scheduler.py

@@ -152,9 +152,54 @@ def _resolve_origin(job: dict) -> Optional[dict]:
     return None
 
 
+def _plugin_cron_env_var(platform_name: str) -> str:
+    """Return the cron home-channel env var registered by a plugin platform.
+
+    Falls through the platform registry so plugins that set
+    ``cron_deliver_env_var`` on their ``PlatformEntry`` get cron delivery
+    support without editing this module.
+    """
+    try:
+        from hermes_cli.plugins import discover_plugins
+        discover_plugins()  # idempotent
+        from gateway.platform_registry import platform_registry
+        entry = platform_registry.get(platform_name.lower())
+        if entry and entry.cron_deliver_env_var:
+            return entry.cron_deliver_env_var
+    except Exception:
+        pass
+    return ""
+
+
+def _is_known_delivery_platform(platform_name: str) -> bool:
+    """Whether ``platform_name`` is a valid cron delivery target.
+
+    Hardcoded built-ins in ``_KNOWN_DELIVERY_PLATFORMS`` are checked first;
+    plugin platforms registered via ``PlatformEntry`` are accepted if they
+    provide a ``cron_deliver_env_var``.
+    """
+    name = platform_name.lower()
+    if name in _KNOWN_DELIVERY_PLATFORMS:
+        return True
+    return bool(_plugin_cron_env_var(name))
+
+
+def _resolve_home_env_var(platform_name: str) -> str:
+    """Return the env var name for a platform's cron home channel.
+
+    Built-in platforms are in ``_HOME_TARGET_ENV_VARS``; plugin platforms are
+    resolved from the platform registry.
+    """
+    name = platform_name.lower()
+    env_var = _HOME_TARGET_ENV_VARS.get(name)
+    if env_var:
+        return env_var
+    return _plugin_cron_env_var(name)
+
+
 def _get_home_target_chat_id(platform_name: str) -> str:
     """Return the configured home target chat/room ID for a delivery platform."""
-    env_var = _HOME_TARGET_ENV_VARS.get(platform_name.lower())
+    env_var = _resolve_home_env_var(platform_name)
     if not env_var:
         return ""
     value = os.getenv(env_var, "")
@@ -167,7 +212,7 @@ def _get_home_target_chat_id(platform_name: str) -> str:
 
 def _get_home_target_thread_id(platform_name: str) -> Optional[str]:
     """Return the optional thread/topic ID for a platform home target."""
-    env_var = _HOME_TARGET_ENV_VARS.get(platform_name.lower())
+    env_var = _resolve_home_env_var(platform_name)
     if not env_var:
         return None
     value = os.getenv(f"{env_var}_THREAD_ID", "").strip()
@@ -178,6 +223,24 @@ def _get_home_target_thread_id(platform_name: str) -> Optional[str]:
     return value or None
 
 
+def _iter_home_target_platforms():
+    """Iterate built-in + plugin platform names that expose a home channel.
+
+    Used by the ``deliver=origin`` fallback when the job has no origin.
+    """
+    for name in _HOME_TARGET_ENV_VARS:
+        yield name
+    try:
+        from hermes_cli.plugins import discover_plugins
+        discover_plugins()  # idempotent
+        from gateway.platform_registry import platform_registry
+        for entry in platform_registry.plugin_entries():
+            if entry.cron_deliver_env_var and entry.name not in _HOME_TARGET_ENV_VARS:
+                yield entry.name
+    except Exception:
+        pass
+
+
 def _resolve_single_delivery_target(job: dict, deliver_value: str) -> Optional[dict]:
     """Resolve one concrete auto-delivery target for a cron job."""
 
@@ -195,7 +258,7 @@ def _resolve_single_delivery_target(job: dict, deliver_value: str) -> Optional[d
             }
         # Origin missing (e.g. job created via API/script) — try each
         # platform's home channel as a fallback instead of silently dropping.
-        for platform_name in _HOME_TARGET_ENV_VARS:
+        for platform_name in _iter_home_target_platforms():
             chat_id = _get_home_target_chat_id(platform_name)
             if chat_id:
                 logger.info(
@@ -251,7 +314,7 @@ def _resolve_single_delivery_target(job: dict, deliver_value: str) -> Optional[d
             "thread_id": origin.get("thread_id"),
         }
 
-    if platform_name.lower() not in _KNOWN_DELIVERY_PLATFORMS:
+    if not _is_known_delivery_platform(platform_name):
         return None
     chat_id = _get_home_target_chat_id(platform_name)
     if not chat_id: