mcp(fix): Guard entrypoint against missing fastmcp dependency

why: The `libtmux-mcp` console script is always installed via
`[project.scripts]`, but `fastmcp` is only declared in
`[project.optional-dependencies]`. A plain `pip install libtmux` installs
the CLI entrypoint, but invoking it crashes with `ModuleNotFoundError`.
what:
- Catch `ImportError` in `main()` and print a helpful install message
  directing users to `pip install libtmux[mcp]`

Author: tony

src/libtmux/mcp/__init__.py

@@ -5,6 +5,16 @@
 
 def main() -> None:
     """Entry point for the libtmux MCP server."""
-    from libtmux.mcp.server import run_server
+    try:
+        from libtmux.mcp.server import run_server
+    except ImportError:
+        import sys
+
+        print(
+            "libtmux MCP server requires fastmcp. "
+            "Install with: pip install libtmux[mcp]",
+            file=sys.stderr,
+        )
+        raise SystemExit(1) from None
 
     run_server()