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>

tests/test_plugins.py

@@ -226,6 +226,42 @@ class TestPluginHooks:
         # Should not raise despite 1/0
         mgr.invoke_hook("post_tool_call", tool_name="x", args={}, result="r", task_id="")
 
+    def test_hook_return_values_collected(self, tmp_path, monkeypatch):
+        """invoke_hook() collects non-None return values from callbacks."""
+        plugins_dir = tmp_path / "hermes_test" / "plugins"
+        _make_plugin_dir(
+            plugins_dir, "ctx_plugin",
+            register_body=(
+                'ctx.register_hook("pre_llm_call", '
+                'lambda **kw: {"context": "memory from plugin"})'
+            ),
+        )
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path / "hermes_test"))
+
+        mgr = PluginManager()
+        mgr.discover_and_load()
+
+        results = mgr.invoke_hook("pre_llm_call", session_id="s1", user_message="hi",
+                                  conversation_history=[], is_first_turn=True, model="test")
+        assert len(results) == 1
+        assert results[0] == {"context": "memory from plugin"}
+
+    def test_hook_none_returns_excluded(self, tmp_path, monkeypatch):
+        """invoke_hook() excludes None returns from the result list."""
+        plugins_dir = tmp_path / "hermes_test" / "plugins"
+        _make_plugin_dir(
+            plugins_dir, "none_hook",
+            register_body='ctx.register_hook("post_llm_call", lambda **kw: None)',
+        )
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path / "hermes_test"))
+
+        mgr = PluginManager()
+        mgr.discover_and_load()
+
+        results = mgr.invoke_hook("post_llm_call", session_id="s1",
+                                  user_message="hi", assistant_response="bye", model="test")
+        assert results == []
+
     def test_invalid_hook_name_warns(self, tmp_path, monkeypatch, caplog):
         """Registering an unknown hook name logs a warning."""
         plugins_dir = tmp_path / "hermes_test" / "plugins"