feat: activate plugin lifecycle hooks (pre/post_llm_call, session start/end) (#3542)

The plugin system defined six lifecycle hooks but only pre_tool_call and
post_tool_call were invoked.  This activates the remaining four so that
external plugins (e.g. memory systems) can hook into the conversation
loop without touching core code.

Hook semantics:
- on_session_start: fires once when a new session is created
- pre_llm_call: fires once per turn before the tool-calling loop;
  plugins can return {"context": "..."} to inject into the ephemeral
  system prompt (not cached, not persisted)
- post_llm_call: fires once per turn after the loop completes, with
  user_message and assistant_response for sync/storage
- on_session_end: fires at the end of every run_conversation call

invoke_hook() now returns a list of non-None callback return values,
enabling pre_llm_call context injection while remaining backward
compatible (existing hooks that return None are unaffected).

Salvaged from PR #2823.

Co-authored-by: Nicolò Boschi <boschi1997@gmail.com>

hermes_cli/plugins.py

@@ -385,16 +385,23 @@ class PluginManager:
     # Hook invocation
     # -----------------------------------------------------------------------
 
-    def invoke_hook(self, hook_name: str, **kwargs: Any) -> None:
+    def invoke_hook(self, hook_name: str, **kwargs: Any) -> List[Any]:
         """Call all registered callbacks for *hook_name*.
 
         Each callback is wrapped in its own try/except so a misbehaving
         plugin cannot break the core agent loop.
+
+        Returns a list of non-``None`` return values from callbacks.
+        This allows hooks like ``pre_llm_call`` to contribute context
+        that the agent core can collect and inject.
         """
         callbacks = self._hooks.get(hook_name, [])
+        results: List[Any] = []
         for cb in callbacks:
             try:
-                cb(**kwargs)
+                ret = cb(**kwargs)
+                if ret is not None:
+                    results.append(ret)
             except Exception as exc:
                 logger.warning(
                     "Hook '%s' callback %s raised: %s",
@@ -402,6 +409,7 @@ class PluginManager:
                     getattr(cb, "__name__", repr(cb)),
                     exc,
                 )
+        return results
 
     # -----------------------------------------------------------------------
     # Introspection
@@ -446,9 +454,12 @@ def discover_plugins() -> None:
     get_plugin_manager().discover_and_load()
 
 
-def invoke_hook(hook_name: str, **kwargs: Any) -> None:
-    """Invoke a lifecycle hook on all loaded plugins."""
-    get_plugin_manager().invoke_hook(hook_name, **kwargs)
+def invoke_hook(hook_name: str, **kwargs: Any) -> List[Any]:
+    """Invoke a lifecycle hook on all loaded plugins.
+
+    Returns a list of non-``None`` return values from plugin callbacks.
+    """
+    return get_plugin_manager().invoke_hook(hook_name, **kwargs)
 
 
 def get_plugin_tool_names() -> Set[str]: