mcp(feat[server]): Add version and expand instructions

why: The MCP lifecycle spec shows `serverInfo` with `name`, `title`, and
`version` fields. The server was missing `version`. The instructions
string also lacked the tmux hierarchy model and env var configuration
that help LLMs use tools effectively.
what:
- Add `version` from `libtmux.__about__.__version__`
- Add tmux hierarchy description (Server > Session > Window > Pane)
- Document LIBTMUX_SOCKET env var default for socket_name

Author: tony

src/libtmux/mcp/server.py

@@ -7,14 +7,19 @@
 
 from fastmcp import FastMCP
 
+from libtmux.__about__ import __version__
+
 mcp = FastMCP(
     name="libtmux",
+    version=__version__,
     instructions=(
         "libtmux MCP server for programmatic tmux control. "
+        "tmux hierarchy: Server > Session > Window > Pane. "
         "Use pane_id (e.g. '%1') as the preferred targeting method - "
         "it is globally unique within a tmux server. "
         "Use send_keys to execute commands and capture_pane to read output. "
-        "All tools accept an optional socket_name parameter for multi-server support."
+        "All tools accept an optional socket_name parameter for multi-server "
+        "support (defaults to LIBTMUX_SOCKET env var)."
     ),
 )