fix(sync): address code review issues in manual conflict resolution

Fixes from code review:
- Fix duplicate check in merge logic (use else clause)
- Handle io.EOF gracefully (treat as quit)
- Add quit (q) option to abort resolution early
- Add accept-all (a) option to auto-merge remaining conflicts
- Fix skipped conflicts to keep local version (not auto-merge)
- Handle json.MarshalIndent errors properly
- Fix truncateText to use rune count for UTF-8 safety
- Update help text with new options
- Add UTF-8 and emoji test cases

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/sync_manual_test.go

@@ -45,6 +45,36 @@ func TestTruncateText(t *testing.T) {
 			maxLen: 30,
 			want:   "line1 line2 line3",
 		},
+		{
+			name:   "very short max",
+			input:  "hello world",
+			maxLen: 3,
+			want:   "...",
+		},
+		{
+			name:   "UTF-8 characters preserved",
+			input:  "Hello 世界！This is a test",
+			maxLen: 12,
+			want:   "Hello 世界！...",
+		},
+		{
+			name:   "UTF-8 exact length",
+			input:  "日本語テスト",
+			maxLen: 6,
+			want:   "日本語テスト",
+		},
+		{
+			name:   "UTF-8 truncate",
+			input:  "日本語テストです",
+			maxLen: 6,
+			want:   "日本語...",
+		},
+		{
+			name:   "emoji handling",
+			input:  "Hello 🌍🌎🌏 World",
+			maxLen: 12,
+			want:   "Hello 🌍🌎🌏...",
+		},
 	}
 
 	for _, tt := range tests {
@@ -164,6 +194,14 @@ func TestInteractiveConflictDisplay(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "both nil (edge case)",
+			conflict: InteractiveConflict{
+				IssueID: "test-6",
+				Local:   nil,
+				Remote:  nil,
+			},
+		},
 	}
 
 	for _, tt := range tests {
@@ -177,22 +215,58 @@ func TestInteractiveConflictDisplay(t *testing.T) {
 func TestShowDetailedDiff(t *testing.T) {
 	now := time.Now()
 
-	conflict := InteractiveConflict{
-		IssueID: "test-1",
-		Local: &beads.Issue{
-			ID:        "test-1",
-			Title:     "Local",
-			UpdatedAt: now,
+	tests := []struct {
+		name     string
+		conflict InteractiveConflict
+	}{
+		{
+			name: "both exist",
+			conflict: InteractiveConflict{
+				IssueID: "test-1",
+				Local: &beads.Issue{
+					ID:        "test-1",
+					Title:     "Local",
+					UpdatedAt: now,
+				},
+				Remote: &beads.Issue{
+					ID:        "test-1",
+					Title:     "Remote",
+					UpdatedAt: now,
+				},
+			},
 		},
-		Remote: &beads.Issue{
-			ID:        "test-1",
-			Title:     "Remote",
-			UpdatedAt: now,
+		{
+			name: "local nil",
+			conflict: InteractiveConflict{
+				IssueID: "test-2",
+				Local:   nil,
+				Remote: &beads.Issue{
+					ID:        "test-2",
+					Title:     "Remote",
+					UpdatedAt: now,
+				},
+			},
+		},
+		{
+			name: "remote nil",
+			conflict: InteractiveConflict{
+				IssueID: "test-3",
+				Local: &beads.Issue{
+					ID:        "test-3",
+					Title:     "Local",
+					UpdatedAt: now,
+				},
+				Remote: nil,
+			},
 		},
 	}
 
-	// Just make sure it doesn't panic
-	showDetailedDiff(conflict)
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Just make sure it doesn't panic
+			showDetailedDiff(tt.conflict)
+		})
+	}
 }
 
 func TestPrintResolutionHelp(t *testing.T) {
@@ -272,3 +346,34 @@ func TestInteractiveResolutionMerge(t *testing.T) {
 		t.Errorf("Expected labels to contain 'bug' and 'feature', got %v", merged.Labels)
 	}
 }
+
+func TestInteractiveResolutionChoices(t *testing.T) {
+	// Test InteractiveResolution struct values
+	tests := []struct {
+		name   string
+		choice string
+		issue  *beads.Issue
+	}{
+		{"local", "local", &beads.Issue{ID: "test"}},
+		{"remote", "remote", &beads.Issue{ID: "test"}},
+		{"merged", "merged", &beads.Issue{ID: "test"}},
+		{"skip", "skip", nil},
+		{"quit", "quit", nil},
+		{"accept-all", "accept-all", nil},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			res := InteractiveResolution{Choice: tt.choice, Issue: tt.issue}
+			if res.Choice != tt.choice {
+				t.Errorf("Expected choice %q, got %q", tt.choice, res.Choice)
+			}
+			if tt.issue == nil && res.Issue != nil {
+				t.Errorf("Expected nil issue, got %v", res.Issue)
+			}
+			if tt.issue != nil && res.Issue == nil {
+				t.Errorf("Expected non-nil issue, got nil")
+			}
+		})
+	}
+}