docs(tools): Correct snapshot_pane title example and document pipe_pane stop-case

why: Two result examples in docs/tools/panes.md drifted from the code:

1. snapshot_pane showed "title": "" but the tool coerces any falsy
   title value to None via `title=parts[8] if parts[8] else None`
   in pane_tools.py — so "" is literally unreachable and the only
   truthful values are a non-empty string or null. Verified with a
   live probe against a fresh pane (surfaced "title": "d", tmux's
   default). Readers copying the example would have serialized data
   that could never arrive over MCP.

2. pipe_pane only documented the start-case response string. The
   side-effects paragraph tells readers to call with output_path=null
   to stop, but no example showed the `"Piping stopped for pane %0"`
   response that the tool returns in that branch (pane_tools.py:970).
   Agents toggling the pipe had to guess the return shape.

what:
- Flip snapshot_pane's example title value from "" to null.
- Extend the pipe_pane section with a second example showing the
  output_path=null call and its corresponding "Piping stopped ..."
  response string.

No code changes; all existing result shapes verified against
src/libtmux_mcp/models.py, the _utils.py serializers, and each tool's
return path. Only these two examples needed correction.

Author: tony

docs/tools/panes.md

@@ -223,7 +223,7 @@ Response:
   "pane_mode": null,
   "scroll_position": null,
   "history_size": 142,
-  "title": "",
+  "title": null,
   "pane_current_command": "zsh",
   "pane_current_path": "/home/user/myproject",
   "is_caller": null
@@ -576,12 +576,30 @@ with `start`/`end` to read scrollback.
 }
 ```
 
-Response (string):
+Response (start):
 
 ```text
 Piping pane %0 to /tmp/build.log
 ```
 
+**Stopping the pipe:**
+
+```json
+{
+  "tool": "pipe_pane",
+  "arguments": {
+    "pane_id": "%0",
+    "output_path": null
+  }
+}
+```
+
+Response (stop):
+
+```text
+Piping stopped for pane %0
+```
+
 ```{fastmcp-tool-input} pane_tools.pipe_pane
 ```