feat(gateway): plugin-platform-safe deserialization via resolve_platform_id

Without this, plugin-registered platforms (like molecule-a2a) survive
the runtime hot path but die on daemon restart: SessionSource.from_dict
calls Platform(value) directly, which raises ValueError for any name
not in the closed enum. Restored sessions with plugin-platform origins
either crash the restore loop or silently drop the platform field.

Add resolve_platform_id() — Platform-first, falls back to
PluginPlatformIdentifier ONLY when a currently-loaded plugin actually
claims the name. Bare typos and corrupted state still raise so silent
state-corruption can't slip past restore. Wire it into the three
known from_dict call sites:

- gateway/session.py:SessionSource.from_dict
- gateway/session.py:SessionEntry.from_dict (drops the silent debug-log
  swallow that left platform=None on unknown values)
- gateway/config.py:HomeChannel.from_dict

All existing gateway/test_session.py round-trip tests still pass,
including test_invalid_platform_raises (the narrow fallback preserves
loud-fail semantics for genuinely unknown names).

gateway/config.py

@@ -90,8 +90,9 @@ class HomeChannel:
     
     @classmethod
     def from_dict(cls, data: Dict[str, Any]) -> "HomeChannel":
+        from hermes_cli.plugins import resolve_platform_id
         return cls(
-            platform=Platform(data["platform"]),
+            platform=resolve_platform_id(data["platform"]),
             chat_id=str(data["chat_id"]),
             name=data.get("name", "Home"),
         )

gateway/session.py

@@ -123,8 +123,9 @@ class SessionSource:
     
     @classmethod
     def from_dict(cls, data: Dict[str, Any]) -> "SessionSource":
+        from hermes_cli.plugins import resolve_platform_id
         return cls(
-            platform=Platform(data["platform"]),
+            platform=resolve_platform_id(data["platform"]),
             chat_id=str(data["chat_id"]),
             chat_name=data.get("chat_name"),
             chat_type=data.get("chat_type", "dm"),
@@ -409,10 +410,8 @@ class SessionEntry:
         
         platform = None
         if data.get("platform"):
-            try:
-                platform = Platform(data["platform"])
-            except ValueError as e:
-                logger.debug("Unknown platform value %r: %s", data["platform"], e)
+            from hermes_cli.plugins import resolve_platform_id
+            platform = resolve_platform_id(data["platform"])
         
         return cls(
             session_key=data["session_key"],

hermes_cli/plugins.py

@@ -883,6 +883,32 @@ class PluginPlatformIdentifier:
         return f"PluginPlatformIdentifier({self.value!r})"
 
 
+def resolve_platform_id(value: str):
+    """Resolve a platform-name string to a ``Platform`` enum member or
+    a ``PluginPlatformIdentifier``.
+
+    Used by ``from_dict`` deserializers for ``SessionSource``,
+    ``SessionEntry``, ``HomeChannel`` etc. Without this fallback,
+    daemon restart would lose every session whose platform was
+    contributed by a plugin (e.g., ``molecule-a2a``) because the
+    bare ``Platform(value)`` call raises ``ValueError`` for unknown
+    enum members.
+
+    The fallback is intentionally narrow: an unknown name is only
+    accepted as a plugin identifier if a currently-loaded plugin has
+    actually claimed that name via ``register_platform_adapter``. Bare
+    typos and corrupted state still raise ``ValueError`` as before, so
+    silent state-corruption doesn't slip past restore.
+    """
+    from gateway.config import Platform  # late import to avoid cycle
+    try:
+        return Platform(value)
+    except ValueError:
+        if get_plugin_platform_adapter(value) is not None:
+            return PluginPlatformIdentifier(value)
+        raise
+
+
 def get_plugin_commands() -> Dict[str, dict]:
     """Return the full plugin commands dict (name → {handler, description, plugin}).