fix(sync): handle sync.branch == current branch (GH#519)

When sync.branch is configured to the same branch as the current branch,
git worktree creation fails because the same branch cannot be checked out
in multiple locations.

This fix detects when sync.branch equals the current branch and falls back
to direct commits on the current branch instead of using the worktree-based
approach.

Changes:
- Add IsSyncBranchSameAsCurrent() helper in syncbranch package
- Add GetCurrentBranch() helper function
- Update sync.go to detect this case and skip worktree operations
- Add unit tests for the new functionality

Closes #519

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/syncbranch/worktree.go

@@ -916,3 +916,25 @@ func HasGitRemote(ctx context.Context) bool {
 	}
 	return len(strings.TrimSpace(string(output))) > 0
 }
+
+// GetCurrentBranch returns the name of the current git branch
+func GetCurrentBranch(ctx context.Context) (string, error) {
+	cmd := exec.CommandContext(ctx, "git", "symbolic-ref", "--short", "HEAD")
+	output, err := cmd.Output()
+	if err != nil {
+		return "", fmt.Errorf("failed to get current branch: %w", err)
+	}
+	return strings.TrimSpace(string(output)), nil
+}
+
+// IsSyncBranchSameAsCurrent returns true if the sync branch is the same as the current branch.
+// This is used to detect the case where we can't use a worktree because the branch is already
+// checked out. In this case, we should commit directly to the current branch instead.
+// See: https://github.com/steveyegge/beads/issues/519
+func IsSyncBranchSameAsCurrent(ctx context.Context, syncBranch string) bool {
+	currentBranch, err := GetCurrentBranch(ctx)
+	if err != nil {
+		return false
+	}
+	return currentBranch == syncBranch
+}