feat(calendar): add --add-attendee flag to preserve existing attendees

The --attendees flag replaces all attendees and resets their RSVP status.
This new --add-attendee flag fetches the current event first and merges
new attendees while preserving existing attendee metadata (responseStatus,
displayName, etc).

Features:
- Add AddAttendee field to CalendarUpdateCmd
- Implement mergeAttendees() for deduplication with case-insensitive email matching
- Validation to prevent using --attendees and --add-attendee together
- New attendees get responseStatus="needsAction"

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

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

Author: salmonumbrella

internal/cmd/calendar.go

@@ -269,7 +269,8 @@ type CalendarUpdateCmd struct {
 	To          string `name:"to" help:"New end time (RFC3339; set empty to clear)"`
 	Description string `name:"description" help:"New description (set empty to clear)"`
 	Location    string `name:"location" help:"New location (set empty to clear)"`
-	Attendees   string `name:"attendees" help:"Comma-separated attendee emails (set empty to clear)"`
+	Attendees   string `name:"attendees" help:"Comma-separated attendee emails (replaces all; set empty to clear)"`
+	AddAttendee string `name:"add-attendee" help:"Comma-separated attendee emails to add (preserves existing attendees)"`
 	AllDay      bool   `name:"all-day" help:"All-day event (use date-only in --from/--to)"`
 }
 
@@ -295,6 +296,11 @@ func (c *CalendarUpdateCmd) Run(ctx context.Context, kctx *kong.Context, flags *
 		}
 	}
 
+	// Cannot use both --attendees and --add-attendee at the same time.
+	if flagProvided(kctx, "attendees") && flagProvided(kctx, "add-attendee") {
+		return usage("cannot use both --attendees and --add-attendee; use --attendees to replace all, or --add-attendee to add")
+	}
+
 	patch := &calendar.Event{}
 	changed := false
 	if flagProvided(kctx, "summary") {
@@ -321,15 +327,26 @@ func (c *CalendarUpdateCmd) Run(ctx context.Context, kctx *kong.Context, flags *
 		patch.Attendees = buildAttendees(c.Attendees)
 		changed = true
 	}
-	if !changed {
-		return usage("no updates provided")
-	}
 
 	svc, err := newCalendarService(ctx, account)
 	if err != nil {
 		return err
 	}
 
+	// For --add-attendee, fetch current event to preserve existing attendees with metadata.
+	if flagProvided(kctx, "add-attendee") {
+		existing, err := svc.Events.Get(calendarID, eventID).Do()
+		if err != nil {
+			return fmt.Errorf("failed to fetch current event: %w", err)
+		}
+		patch.Attendees = mergeAttendees(existing.Attendees, c.AddAttendee)
+		changed = true
+	}
+
+	if !changed {
+		return usage("no updates provided")
+	}
+
 	updated, err := svc.Events.Patch(calendarID, eventID, patch).Do()
 	if err != nil {
 		return err
@@ -592,6 +609,39 @@ func buildAttendees(csv string) []*calendar.EventAttendee {
 	return out
 }
 
+// mergeAttendees preserves existing attendees (with all their metadata like responseStatus)
+// and adds new attendees from the CSV string. Duplicates (by email) are skipped.
+func mergeAttendees(existing []*calendar.EventAttendee, addCSV string) []*calendar.EventAttendee {
+	newEmails := splitCSV(addCSV)
+	if len(newEmails) == 0 {
+		return existing
+	}
+
+	// Build a set of existing emails for deduplication
+	existingEmails := make(map[string]bool, len(existing))
+	for _, a := range existing {
+		if a != nil && a.Email != "" {
+			existingEmails[strings.ToLower(a.Email)] = true
+		}
+	}
+
+	// Start with existing attendees (preserving all metadata)
+	out := make([]*calendar.EventAttendee, 0, len(existing)+len(newEmails))
+	out = append(out, existing...)
+
+	// Add new attendees that don't already exist
+	for _, email := range newEmails {
+		if !existingEmails[strings.ToLower(email)] {
+			out = append(out, &calendar.EventAttendee{
+				Email:          email,
+				ResponseStatus: "needsAction",
+			})
+			existingEmails[strings.ToLower(email)] = true
+		}
+	}
+	return out
+}
+
 func splitCSV(s string) []string {
 	s = strings.TrimSpace(s)
 	if s == "" {

internal/cmd/calendar_add_attendee_test.go

@@ -0,0 +1,99 @@
+package cmd
+
+import (
+	"testing"
+
+	"google.golang.org/api/calendar/v3"
+)
+
+func TestMergeAttendees(t *testing.T) {
+	tests := []struct {
+		name     string
+		existing []*calendar.EventAttendee
+		addCSV   string
+		wantLen  int
+	}{
+		{
+			name:     "add to empty list",
+			existing: nil,
+			addCSV:   "a@test.com,b@test.com",
+			wantLen:  2,
+		},
+		{
+			name: "add to existing list",
+			existing: []*calendar.EventAttendee{
+				{Email: "existing@test.com", ResponseStatus: "accepted"},
+			},
+			addCSV:  "new@test.com",
+			wantLen: 2,
+		},
+		{
+			name: "skip duplicates case-insensitive",
+			existing: []*calendar.EventAttendee{
+				{Email: "Existing@Test.com", ResponseStatus: "accepted"},
+			},
+			addCSV:  "existing@test.com,new@test.com",
+			wantLen: 2,
+		},
+		{
+			name: "preserve existing metadata",
+			existing: []*calendar.EventAttendee{
+				{Email: "alice@test.com", ResponseStatus: "accepted", DisplayName: "Alice"},
+			},
+			addCSV:  "bob@test.com",
+			wantLen: 2,
+		},
+		{
+			name: "empty add string",
+			existing: []*calendar.EventAttendee{
+				{Email: "keep@test.com", ResponseStatus: "accepted"},
+			},
+			addCSV:  "",
+			wantLen: 1,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := mergeAttendees(tt.existing, tt.addCSV)
+			if len(got) != tt.wantLen {
+				t.Errorf("mergeAttendees() returned %d attendees, want %d", len(got), tt.wantLen)
+			}
+
+			if tt.name == "preserve existing metadata" && len(got) > 0 {
+				found := false
+				for _, a := range got {
+					if a.Email == "alice@test.com" {
+						found = true
+						if a.ResponseStatus != "accepted" {
+							t.Errorf("existing attendee lost responseStatus")
+						}
+						if a.DisplayName != "Alice" {
+							t.Errorf("existing attendee lost displayName")
+						}
+					}
+				}
+				if !found {
+					t.Errorf("existing attendee alice@test.com not found in result")
+				}
+			}
+		})
+	}
+}
+
+func TestMergeAttendeesNewHaveNeedsAction(t *testing.T) {
+	existing := []*calendar.EventAttendee{
+		{Email: "existing@test.com", ResponseStatus: "accepted"},
+	}
+	got := mergeAttendees(existing, "new@test.com")
+
+	for _, a := range got {
+		if a.Email == "new@test.com" {
+			if a.ResponseStatus != "needsAction" {
+				t.Errorf("new attendee should have responseStatus=needsAction, got %q", a.ResponseStatus)
+			}
+			return
+		}
+	}
+	t.Error("new attendee not found in result")
+}