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>

website/docs/user-guide/features/plugins.md

@@ -52,10 +52,10 @@ Plugins can register callbacks for these lifecycle events. See the **[Event Hook
 |------|-----------|
 | `pre_tool_call` | Before any tool executes |
 | `post_tool_call` | After any tool returns |
-| `pre_llm_call` | Before LLM API request *(planned)* |
-| `post_llm_call` | After LLM API response *(planned)* |
-| `on_session_start` | Session begins *(planned)* |
-| `on_session_end` | Session ends *(planned)* |
+| `pre_llm_call` | Once per turn, before the LLM loop — can return `{"context": "..."}` to inject into the system prompt |
+| `post_llm_call` | Once per turn, after the LLM loop completes |
+| `on_session_start` | New session created (first turn only) |
+| `on_session_end` | End of every `run_conversation` call |
 
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