mcp(docs[tools/sessions]): note SessionInfo.active_pane_id surfaces the new pane id

The ``create_session`` response gained an ``active_pane_id`` field
earlier in this branch (commit ``3f092bb``), but the docs still
showed the pre-change response shape — readers landing on
``sessions.md`` had no signal that this field exists or what
they'd do with it.

Updates the example response to include the field, then adds a
brief tip explaining the workflow win: callers can target
subsequent ``send_keys`` / ``split_window`` / ``capture_pane``
calls against ``active_pane_id`` directly without a follow-up
``list_panes`` round-trip. Saves one MCP call in the most common
"create a session, then act on it" workflow that prompts like
``build_dev_workspace`` already exploit.

The full field surface is auto-rendered in
``docs/reference/api/models.md`` via the ``fastmcp_model_classes``
registration — this commit just adds the narrative discoverability
that explains why the field exists.

Author: tony

docs/tools/sessions.md

@@ -177,10 +177,21 @@ Response:
   "session_name": "dev",
   "window_count": 1,
   "session_attached": "0",
-  "session_created": "1774521872"
+  "session_created": "1774521872",
+  "active_pane_id": "%0"
 }
 ```
 
+```{tip}
+The returned ``active_pane_id`` is the pane ID (``%N``) of the
+session's initial pane. It's guaranteed non-``None`` immediately
+after ``create_session`` (libtmux always creates the session with
+one initial pane), so you can target subsequent ``send_keys`` /
+``split_window`` / ``capture_pane`` calls directly without a
+follow-up {tooliconl}`list-panes` round-trip — saving an MCP call
+in the most common "new session, then act on it" workflow.
+```
+
 ```{fastmcp-tool-input} server_tools.create_session
 ```