feat(daemon): Add --global flag for multi-repo support

Implements bd-121: Global daemon with system-wide socket

Changes:
- Add --global flag to daemon command
- Use ~/.beads/bd.sock when --global is set
- Skip git repo validation for global daemon
- Update daemon discovery to check ~/.beads/ as fallback
- Both Go CLI and Python MCP client check global socket
- Update all tests to pass global parameter

Benefits:
- Single daemon serves all repos on system
- No per-repo daemon management needed
- Better resource usage for users with many repos
- Automatic fallback when local daemon not running

Usage:
  bd daemon --global         # Start global daemon
  bd daemon --status --global # Check global status
  bd daemon --stop --global   # Stop global daemon

Related: bd-73 (multi-repo epic)
Amp-Thread-ID: https://ampcode.com/threads/T-ea606216-b886-4af0-bba8-56d000362d01
Co-authored-by: Amp <amp@ampcode.com>

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

@@ -640,7 +640,7 @@ def create_bd_client(
             from pathlib import Path
 
             # Check if daemon socket exists before creating client
-            # Walk up from working_dir to find .beads/bd.sock
+            # Walk up from working_dir to find .beads/bd.sock, then check global
             search_dir = Path(working_dir) if working_dir else Path.cwd()
             socket_found = False
 
@@ -652,16 +652,22 @@ def create_bd_client(
                     if sock_path.exists():
                         socket_found = True
                         break
-                    # Found .beads but no socket - daemon not running
+                    # Found .beads but no socket - check global before giving up
                     break
 
                 # Move up one directory
                 parent = current.parent
                 if parent == current:
-                    # Reached filesystem root
+                    # Reached filesystem root - check global
                     break
                 current = parent
 
+            # If no local socket, check global daemon socket at ~/.beads/bd.sock
+            if not socket_found:
+                global_sock_path = Path.home() / ".beads" / "bd.sock"
+                if global_sock_path.exists():
+                    socket_found = True
+
             if socket_found:
                 # Daemon is running, use it
                 client = BdDaemonClient(