fix: Validate sync-branch at config-time and runtime (closes #1166) (#1168)

* fix(config): validate sync-branch at config time

Add sync-branch validation to validateYamlConfigValue() to reject
main/master at config time, preventing the validation bypass in GH#1166.

- Add case for sync-branch and sync.branch keys
- Inline validation logic to avoid import cycle (config <-> syncbranch)
- Add unit tests for rejection (main/master) and acceptance (valid names)

Part of: #1166

* fix(sync): add runtime guard for sync-branch == current-branch

Add dynamic runtime check before worktree operations to catch cases
where sync-branch matches the current branch. This provides defense
in depth for manual YAML edits, pre-fix configs, or non-main/master
branch names (trunk, develop, production, etc.).

- Check IsSyncBranchSameAsCurrent() after hasSyncBranchConfig is set
- Position check BEFORE worktree entry (CWD changes inside worktree)
- Add integration test TestSync_FailsWhenOnSyncBranch

Part of: #1166

* docs: note main/master restriction in sync-branch FAQ

Clarifies that git worktrees cannot checkout the same branch in
multiple locations, so main/master cannot be used as sync branch.

cmd/bd/sync.go

@@ -273,6 +273,17 @@ The --full flag provides the legacy full sync behavior for backwards compatibili
 		}
 		hasSyncBranchConfig := syncBranchName != ""
 
+		// GH#1166: Block sync if currently on the sync branch
+		// This must happen BEFORE worktree operations - after entering a worktree,
+		// GetCurrentBranch() would return the worktree's branch, not the original.
+		if hasSyncBranchConfig {
+			if syncbranch.IsSyncBranchSameAsCurrent(ctx, syncBranchName) {
+				FatalError("Cannot sync to '%s': it's your current branch. "+
+					"Checkout a different branch first, or use a dedicated sync branch like 'beads-sync'.",
+					syncBranchName)
+			}
+		}
+
 		// bd-wayc3: Check for redirect + sync-branch incompatibility
 		// Redirect and sync-branch are mutually exclusive:
 		// - Redirect says: "My database is in another repo (I am a client)"

cmd/bd/sync_modes_test.go

@@ -773,3 +773,82 @@ func TestExportOnlySync(t *testing.T) {
 	}
 	t.Log("✓ issues.jsonl is committed after export-only sync")
 }
+
+// TestSync_FailsWhenOnSyncBranch verifies that bd sync fails gracefully
+// when the current branch matches the configured sync branch.
+// This is the runtime guard for GH#1166 - prevents bd sync from attempting
+// to create a worktree for a branch that's already checked out.
+//
+// The issue: If sync.branch = "main" and user is on "main", bd sync would
+// try to create a worktree for "main" which fails because it's already checked out.
+// Worse, some code paths could commit all staged files instead of just .beads/.
+func TestSync_FailsWhenOnSyncBranch(t *testing.T) {
+	ctx := context.Background()
+	tmpDir, cleanup := setupGitRepo(t)
+	defer cleanup()
+
+	// Get current branch name (should be "main" from setupGitRepo)
+	currentBranch, err := syncbranch.GetCurrentBranch(ctx)
+	if err != nil {
+		t.Fatalf("failed to get current branch: %v", err)
+	}
+	t.Logf("Current branch: %s", currentBranch)
+
+	// Test 1: IsSyncBranchSameAsCurrent returns true when branch matches
+	if !syncbranch.IsSyncBranchSameAsCurrent(ctx, currentBranch) {
+		t.Error("IsSyncBranchSameAsCurrent should return true when sync branch matches current branch")
+	}
+	t.Log("✓ IsSyncBranchSameAsCurrent correctly detects same-branch condition")
+
+	// Test 2: IsSyncBranchSameAsCurrent returns false for different branch
+	if syncbranch.IsSyncBranchSameAsCurrent(ctx, "beads-sync") {
+		t.Error("IsSyncBranchSameAsCurrent should return false for different branch name")
+	}
+	t.Log("✓ IsSyncBranchSameAsCurrent correctly allows different branch names")
+
+	// Test 3: Setup sync.branch config to match current branch (the problematic state)
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatalf("failed to create .beads dir: %v", err)
+	}
+
+	dbPath := filepath.Join(beadsDir, "beads.db")
+	testStore, err := sqlite.New(ctx, dbPath)
+	if err != nil {
+		t.Fatalf("failed to create store: %v", err)
+	}
+	defer testStore.Close()
+
+	// Configure sync.branch to match current branch (this is the bug condition)
+	if err := testStore.SetConfig(ctx, "sync.branch", currentBranch); err != nil {
+		t.Fatalf("failed to set sync.branch: %v", err)
+	}
+	t.Logf("✓ Configured sync.branch = %s (matches current branch)", currentBranch)
+
+	// Verify the config is stored correctly
+	syncBranchValue, err := testStore.GetConfig(ctx, "sync.branch")
+	if err != nil {
+		t.Fatalf("failed to get sync.branch config: %v", err)
+	}
+	if syncBranchValue != currentBranch {
+		t.Errorf("sync.branch config = %q, want %q", syncBranchValue, currentBranch)
+	}
+
+	// Test 4: The runtime guard logic (same as in sync.go)
+	// This simulates what happens in the sync command
+	syncBranchName := syncBranchValue
+	hasSyncBranchConfig := syncBranchName != ""
+
+	if hasSyncBranchConfig {
+		if syncbranch.IsSyncBranchSameAsCurrent(ctx, syncBranchName) {
+			// This is the expected behavior - we caught the misconfiguration
+			t.Log("✓ Runtime guard correctly detected sync-branch = current-branch condition")
+		} else {
+			t.Error("FAIL: Runtime guard did not detect sync-branch = current-branch condition")
+		}
+	} else {
+		t.Error("FAIL: sync.branch config should be set")
+	}
+
+	t.Log("✓ TestSync_FailsWhenOnSyncBranch PASSED")
+}