fix(beads-mcp): prevent subprocess stdin inheritance breaking MCP protocol

When running as an MCP server, subprocesses must not inherit stdin because
MCP uses stdin for JSON-RPC protocol communication. Inherited stdin causes
subprocesses to block indefinitely waiting for input or steal bytes from
the MCP protocol stream.

Added stdin=subprocess.DEVNULL (or asyncio.subprocess.DEVNULL for async)
to all subprocess calls:
- server.py: _resolve_workspace_root() git subprocess
- tools.py: _resolve_workspace_root() git subprocess
- bd_client.py: _run_command(), _check_version(), add_dependency(),
  quickstart(), and init() async subprocess calls

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

integrations/beads-mcp/src/beads_mcp/server.py

@@ -218,6 +218,7 @@ def _resolve_workspace_root(path: str) -> str:
             text=True,
             check=False,
             shell=sys.platform == "win32",
+            stdin=subprocess.DEVNULL,  # Prevent inheriting MCP's stdin
         )
         if result.returncode == 0:
             return result.stdout.strip()