fix(doctor): recognize lowercase 's' skip-worktree flag (#931)

* fix(doctor): handle 's' status in combined git flags

Problem:
- Git status detection failed when 's' was combined with other flags
- Branch synchronization checks produced incorrect results due to missing flag parsing

Solution:
- Update detection logic to correctly identify the 's' status within combined flag strings

Impact:
- Ensures branch synchronization state is accurately reported during doctor checks

* test(doctor): add unit tests for git flag parsing

- Extract git flag parsing logic into parseGitLsFilesFlag helper
- Add unit tests for git flag parsing logic

Coverage: Git flag parsing in sync_branch.go

cmd/bd/doctor/fix/sync_branch_test.go

@@ -0,0 +1,46 @@
+package fix
+
+import "testing"
+
+func TestParseGitLsFilesFlag(t *testing.T) {
+	tests := map[string]struct {
+		flag              byte
+		wantHasAnyFlag    bool
+		wantHasSkipWorktree bool
+	}{
+		"normal tracked file (H)": {
+			flag:                'H',
+			wantHasAnyFlag:      false,
+			wantHasSkipWorktree: false,
+		},
+		"assume-unchanged only (h)": {
+			flag:                'h',
+			wantHasAnyFlag:      true,
+			wantHasSkipWorktree: false,
+		},
+		"skip-worktree only (S)": {
+			flag:                'S',
+			wantHasAnyFlag:      true,
+			wantHasSkipWorktree: true,
+		},
+		"both flags set (s)": {
+			flag:                's',
+			wantHasAnyFlag:      true,
+			wantHasSkipWorktree: true,
+		},
+	}
+
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			gotHasAnyFlag, gotHasSkipWorktree := parseGitLsFilesFlag(tt.flag)
+
+			if gotHasAnyFlag != tt.wantHasAnyFlag {
+				t.Errorf("hasAnyFlag = %v, want %v", gotHasAnyFlag, tt.wantHasAnyFlag)
+			}
+
+			if gotHasSkipWorktree != tt.wantHasSkipWorktree {
+				t.Errorf("hasSkipWorktree = %v, want %v", gotHasSkipWorktree, tt.wantHasSkipWorktree)
+			}
+		})
+	}
+}