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>

cmd/bd/daemon_test.go

@@ -23,7 +23,7 @@ func TestGetPIDFilePath(t *testing.T) {
 	defer func() { dbPath = oldDBPath }()
 
 	dbPath = filepath.Join(tmpDir, ".beads", "test.db")
-	pidFile, err := getPIDFilePath()
+	pidFile, err := getPIDFilePath(false) // test local daemon
 	if err != nil {
 		t.Fatalf("getPIDFilePath failed: %v", err)
 	}
@@ -65,7 +65,7 @@ func TestGetLogFilePath(t *testing.T) {
 			defer func() { dbPath = oldDBPath }()
 			dbPath = tt.dbPath
 
-			result, err := getLogFilePath(tt.userPath)
+			result, err := getLogFilePath(tt.userPath, false) // test local daemon
 			if err != nil {
 				t.Fatalf("getLogFilePath failed: %v", err)
 			}