fix(init): bootstrap from sync-branch when configured (bd-0is)

When sync-branch is configured in config.yaml, bd init now reads from
that branch (origin/<branch> first, then local <branch>) instead of
HEAD. This ensures fresh clones correctly import issues from the sync
branch.

Key changes:
- checkGitForIssues() now returns gitRef (third return value)
- New getLocalSyncBranch() reads sync-branch directly from config.yaml
  (not cached global config) to handle test environments where CWD changes
- importFromGit() accepts gitRef parameter to read from correct branch
- Added readFirstIssueFromGit() for prefix auto-detection from git
- Fixed macOS symlink issue: filepath.EvalSymlinks() ensures /var and
  /private/var paths are normalized before filepath.Rel()

Part of GitHub issue #464 (beads deletes issues in multi-clone environments)

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

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

cmd/bd/autoimport_test.go

@@ -250,13 +250,16 @@ func TestCheckGitForIssues_NoGitRepo(t *testing.T) {
 	tmpDir := t.TempDir()
 	t.Chdir(tmpDir)
 
-	count, path := checkGitForIssues()
+	count, path, gitRef := checkGitForIssues()
 	if count != 0 {
 		t.Errorf("Expected 0 issues, got %d", count)
 	}
 	if path != "" {
 		t.Errorf("Expected empty path, got %s", path)
 	}
+	if gitRef != "" {
+		t.Errorf("Expected empty gitRef, got %s", gitRef)
+	}
 }
 
 func TestCheckGitForIssues_NoBeadsDir(t *testing.T) {
@@ -264,7 +267,7 @@ func TestCheckGitForIssues_NoBeadsDir(t *testing.T) {
 	tmpDir := t.TempDir()
 	t.Chdir(tmpDir)
 
-	count, path := checkGitForIssues()
+	count, path, _ := checkGitForIssues()
 	if count != 0 || path != "" {
 		t.Logf("No .beads dir: count=%d, path=%s (expected 0, empty)", count, path)
 	}