fix(sync): respect sync.branch config when no upstream tracking (#640)

* fix(sync): respect sync.branch config when no upstream tracking

When sync.branch is explicitly configured, bd sync should use worktree-based
sync even if the current branch has no upstream tracking. This fixes the
issue where jj (Jujutsu) colocated repos and git worktrees without upstream
would incorrectly fall back to --from-main mode.

The fix checks for sync.branch configuration BEFORE the upstream check,
allowing the configured sync branch to take precedence.

Affected workflows:
- jj colocated repos (always use detached HEAD)
- Git worktrees not tracking a remote
- Temporary checkouts for testing

Fixes #638

* test(sync): add regression tests for sync.branch priority over upstream check

Add tests verifying that when sync.branch is configured, bd sync does NOT
fall back to --from-main mode even without upstream tracking. This covers:

- sync.branch configured without upstream (should use worktree sync)
- No sync.branch and no upstream (should fallback to from-main)
- Detached HEAD with sync.branch (jj workflow, should use worktree sync)

These tests ensure the fix for GH#638 doesn't regress.

---------

Co-authored-by: Charles P. Cross <cpdata@users.noreply.github.com>

cmd/bd/sync.go

@@ -179,9 +179,20 @@ Use --merge to merge the sync branch back to main branch.`,
 			os.Exit(1)
 		}
 
+		// GH#638: Check sync.branch BEFORE upstream check
+		// When sync.branch is configured, we should use worktree-based sync even if
+		// the current branch has no upstream (e.g., detached HEAD in jj, git worktrees)
+		var hasSyncBranchConfig bool
+		if err := ensureStoreActive(); err == nil && store != nil {
+			if syncBranch, _ := syncbranch.Get(ctx, store); syncBranch != "" {
+				hasSyncBranchConfig = true
+			}
+		}
+
 		// Preflight: check for upstream tracking
 		// If no upstream, automatically switch to --from-main mode (gt-ick9: ephemeral branch support)
-		if !noPull && !gitHasUpstream() {
+		// GH#638: Skip this fallback if sync.branch is explicitly configured
+		if !noPull && !gitHasUpstream() && !hasSyncBranchConfig {
 			if hasGitRemote(ctx) {
 				// Remote exists but no upstream - use from-main mode
 				fmt.Println("→ No upstream configured, using --from-main mode")