Add bd doctor check and fix for missing sync.branch config (bd-rsua)

Problem: Existing beads repositories initialized before commit a4c38d5 don't have sync.branch configured. This causes 'bd sync --status' to fail with a confusing error.

Solution: Added new check in 'bd doctor' that detects when sync.branch is not configured and provides automatic fix via 'bd doctor --fix'. The fix automatically sets sync.branch to the current branch using 'git symbolic-ref --short HEAD'.

Changes:
- Added checkSyncBranchConfig() function in doctor.go
- Created fix/sync_branch.go with SyncBranchConfig() fix handler
- Added comprehensive test coverage in doctor_test.go
- Integrated check into applyFixes() switch statement

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

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

cmd/bd/doctor/fix/sync_branch.go

@@ -0,0 +1,43 @@
+package fix
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// SyncBranchConfig fixes missing sync.branch configuration by auto-setting it to the current branch
+func SyncBranchConfig(path string) error {
+	if err := validateBeadsWorkspace(path); err != nil {
+		return err
+	}
+
+	// Get current branch
+	cmd := exec.Command("git", "symbolic-ref", "--short", "HEAD")
+	cmd.Dir = path
+	output, err := cmd.Output()
+	if err != nil {
+		return fmt.Errorf("failed to get current branch: %w", err)
+	}
+
+	currentBranch := strings.TrimSpace(string(output))
+	if currentBranch == "" {
+		return fmt.Errorf("current branch is empty")
+	}
+
+	// Get bd binary
+	bdBinary, err := getBdBinary()
+	if err != nil {
+		return err
+	}
+
+	// Set sync.branch using bd config set
+	setCmd := exec.Command(bdBinary, "config", "set", "sync.branch", currentBranch)
+	setCmd.Dir = path
+	if output, err := setCmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("failed to set sync.branch: %w\nOutput: %s", err, string(output))
+	}
+
+	fmt.Printf("  Set sync.branch = %s\n", currentBranch)
+	return nil
+}