fix(sync): default noGitHistory=true for --from-main mode (#418)

Fixes #417: When using --from-main mode (either explicitly or auto-detected),
git history backfill now defaults to disabled. This prevents creating
incorrect deletion records for locally-created beads that don't exist in
main's git history.

Changes:
- Add resolveNoGitHistoryForFromMain() helper function
- Apply noGitHistory=true for both explicit and auto-detected from-main mode
- Add comprehensive unit tests

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

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

cmd/bd/sync.go

@@ -68,6 +68,9 @@ Use --merge to merge the sync branch back to main branch.`,
 			daemonClient = nil
 		}
 
+		// Resolve noGitHistory based on fromMain (fixes #417)
+		noGitHistory = resolveNoGitHistoryForFromMain(fromMain, noGitHistory)
+
 		// Find JSONL path
 		jsonlPath := findJSONLPath()
 		if jsonlPath == "" {
@@ -170,7 +173,8 @@ Use --merge to merge the sync branch back to main branch.`,
 			if hasGitRemote(ctx) {
 				// Remote exists but no upstream - use from-main mode
 				fmt.Println("→ No upstream configured, using --from-main mode")
-				if err := doSyncFromMain(ctx, jsonlPath, renameOnImport, dryRun, noGitHistory); err != nil {
+				// Force noGitHistory=true for auto-detected from-main mode (fixes #417)
+				if err := doSyncFromMain(ctx, jsonlPath, renameOnImport, dryRun, true); err != nil {
 					fmt.Fprintf(os.Stderr, "Error: %v\n", err)
 					os.Exit(1)
 				}
@@ -1495,3 +1499,14 @@ func sanitizeJSONLWithDeletions(jsonlPath string) (*SanitizeResult, error) {
 
 	return result, nil
 }
+
+// resolveNoGitHistoryForFromMain returns the resolved noGitHistory value for sync operations.
+// When syncing from main (--from-main), noGitHistory is forced to true to prevent creating
+// incorrect deletion records for locally-created beads that don't exist on main.
+// See: https://github.com/steveyegge/beads/issues/417
+func resolveNoGitHistoryForFromMain(fromMain, noGitHistory bool) bool {
+	if fromMain {
+		return true
+	}
+	return noGitHistory
+}

cmd/bd/sync_test.go

@@ -1105,3 +1105,50 @@ func TestHashBasedStalenessDetection_bd_f2f(t *testing.T) {
 		t.Error("hasJSONLChanged should return false after hash is updated to match JSONL")
 	}
 }
+
+// TestResolveNoGitHistoryForFromMain tests that --from-main forces noGitHistory=true
+// to prevent creating incorrect deletion records for locally-created beads.
+// See: https://github.com/steveyegge/beads/issues/417
+func TestResolveNoGitHistoryForFromMain(t *testing.T) {
+	tests := []struct {
+		name         string
+		fromMain     bool
+		noGitHistory bool
+		want         bool
+	}{
+		{
+			name:         "fromMain=true forces noGitHistory=true regardless of flag",
+			fromMain:     true,
+			noGitHistory: false,
+			want:         true,
+		},
+		{
+			name:         "fromMain=true with noGitHistory=true stays true",
+			fromMain:     true,
+			noGitHistory: true,
+			want:         true,
+		},
+		{
+			name:         "fromMain=false preserves noGitHistory=false",
+			fromMain:     false,
+			noGitHistory: false,
+			want:         false,
+		},
+		{
+			name:         "fromMain=false preserves noGitHistory=true",
+			fromMain:     false,
+			noGitHistory: true,
+			want:         true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := resolveNoGitHistoryForFromMain(tt.fromMain, tt.noGitHistory)
+			if got != tt.want {
+				t.Errorf("resolveNoGitHistoryForFromMain(%v, %v) = %v, want %v",
+					tt.fromMain, tt.noGitHistory, got, tt.want)
+			}
+		})
+	}
+}