feat: block archived agents in squad APIs and add is_archived to member response

- CreateSquad: reject archived agent as leader
- UpdateSquad: reject archived agent as new leader
- AddSquadMember: reject adding archived agent
- ListSquadMembers: add is_archived field to SquadMemberResponse

Co-authored-by: multica-agent <github@multica.ai>

server/internal/handler/squad.go

@@ -37,6 +37,7 @@ type SquadMemberResponse struct {
 	MemberID   string `json:"member_id"`
 	Role       string `json:"role"`
 	CreatedAt  string `json:"created_at"`
+	IsArchived bool   `json:"is_archived"`
 }
 
 // ── Converters ──────────────────────────────────────────────────────────────
@@ -149,7 +150,7 @@ func (h *Handler) CreateSquad(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Validate leader is an agent in this workspace.
-	_, err := h.Queries.GetAgentInWorkspace(r.Context(), db.GetAgentInWorkspaceParams{
+	leader, err := h.Queries.GetAgentInWorkspace(r.Context(), db.GetAgentInWorkspaceParams{
 		ID:          leaderUUID,
 		WorkspaceID: wsUUID,
 	})
@@ -157,6 +158,10 @@ func (h *Handler) CreateSquad(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, "leader must be a valid agent in this workspace")
 		return
 	}
+	if leader.ArchivedAt.Valid {
+		writeError(w, http.StatusBadRequest, "cannot use an archived agent as squad leader")
+		return
+	}
 
 	squad, err := h.Queries.CreateSquad(r.Context(), db.CreateSquadParams{
 		WorkspaceID: wsUUID,
@@ -237,12 +242,17 @@ func (h *Handler) UpdateSquad(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 		// Validate new leader is an agent in workspace.
-		if _, err := h.Queries.GetAgentInWorkspace(r.Context(), db.GetAgentInWorkspaceParams{
+		newLeader, err := h.Queries.GetAgentInWorkspace(r.Context(), db.GetAgentInWorkspaceParams{
 			ID: lid, WorkspaceID: wsUUID,
-		}); err != nil {
+		})
+		if err != nil {
 			writeError(w, http.StatusBadRequest, "leader must be a valid agent in this workspace")
 			return
 		}
+		if newLeader.ArchivedAt.Valid {
+			writeError(w, http.StatusBadRequest, "cannot use an archived agent as squad leader")
+			return
+		}
 		// Ensure new leader is a squad member; auto-add if not.
 		isMember, _ := h.Queries.IsSquadMember(r.Context(), db.IsSquadMemberParams{
 			SquadID: squad.ID, MemberType: "agent", MemberID: lid,
@@ -323,6 +333,11 @@ func (h *Handler) ListSquadMembers(w http.ResponseWriter, r *http.Request) {
 	resp := make([]SquadMemberResponse, len(members))
 	for i, m := range members {
 		resp[i] = squadMemberToResponse(m)
+		if m.MemberType == "agent" {
+			if ag, err := h.Queries.GetAgent(r.Context(), m.MemberID); err == nil {
+				resp[i].IsArchived = ag.ArchivedAt.Valid
+			}
+		}
 	}
 	writeJSON(w, http.StatusOK, resp)
 }
@@ -367,12 +382,17 @@ func (h *Handler) AddSquadMember(w http.ResponseWriter, r *http.Request) {
 
 	// Validate the member belongs to this workspace.
 	if req.MemberType == "agent" {
-		if _, err := h.Queries.GetAgentInWorkspace(r.Context(), db.GetAgentInWorkspaceParams{
+		agent, err := h.Queries.GetAgentInWorkspace(r.Context(), db.GetAgentInWorkspaceParams{
 			ID: memberUUID, WorkspaceID: wsUUID,
-		}); err != nil {
+		})
+		if err != nil {
 			writeError(w, http.StatusBadRequest, "agent not found in this workspace")
 			return
 		}
+		if agent.ArchivedAt.Valid {
+			writeError(w, http.StatusBadRequest, "cannot add an archived agent to a squad")
+			return
+		}
 	} else {
 		if _, err := h.Queries.GetMemberByUserAndWorkspace(r.Context(), db.GetMemberByUserAndWorkspaceParams{
 			UserID: memberUUID, WorkspaceID: wsUUID,