feat: add structured labels for agent beads (bd-g7eq)

Add role_type and rig labels to agent beads for filtering queries.

Changes:
- Add RoleType/Rig to CreateArgs and UpdateArgs in RPC protocol
- Auto-add role_type:<value> and rig:<value> labels when creating/updating agents
- Add --role-type and --agent-rig flags to bd create (requires --type=agent)
- Add bd agent backfill-labels command to update existing agent beads

This enables queries like:
  bd list --type=agent --label=role_type:witness
  bd list --type=agent --label=rig:gastown

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

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

internal/rpc/protocol.go

@@ -97,6 +97,9 @@ type CreateArgs struct {
 	CreatedBy string `json:"created_by,omitempty"` // Who created the issue
 	// Molecule type (for swarm coordination)
 	MolType string `json:"mol_type,omitempty"` // swarm, patrol, or work (default)
+	// Agent identity fields (only valid when IssueType == "agent")
+	RoleType string `json:"role_type,omitempty"` // polecat|crew|witness|refinery|mayor|deacon
+	Rig      string `json:"rig,omitempty"`       // Rig name (empty for town-level agents)
 }
 
 // UpdateArgs represents arguments for the update operation
@@ -134,6 +137,9 @@ type UpdateArgs struct {
 	// Agent state fields
 	AgentState   *string `json:"agent_state,omitempty"`   // Agent state (idle|running|stuck|stopped|dead)
 	LastActivity *bool   `json:"last_activity,omitempty"` // If true, update last_activity to now
+	// Agent identity fields
+	RoleType *string `json:"role_type,omitempty"` // polecat|crew|witness|refinery|mayor|deacon
+	Rig      *string `json:"rig,omitempty"`       // Rig name (empty for town-level agents)
 }
 
 // CloseArgs represents arguments for the close operation

internal/rpc/server_issues_epics.go

@@ -115,6 +115,13 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.LastActivity != nil && *a.LastActivity {
 		u["last_activity"] = time.Now()
 	}
+	// Agent identity fields
+	if a.RoleType != nil {
+		u["role_type"] = *a.RoleType
+	}
+	if a.Rig != nil {
+		u["rig"] = *a.Rig
+	}
 	return u
 }
 
@@ -198,6 +205,9 @@ func (s *Server) handleCreate(req *Request) Response {
 		CreatedBy: createArgs.CreatedBy,
 		// Molecule type
 		MolType: types.MolType(createArgs.MolType),
+		// Agent identity fields
+		RoleType: createArgs.RoleType,
+		Rig:      createArgs.Rig,
 	}
 	
 	// Check if any dependencies are discovered-from type
@@ -283,6 +293,28 @@ func (s *Server) handleCreate(req *Request) Response {
 		}
 	}
 
+	// Auto-add role_type/rig labels for agent beads (enables filtering queries)
+	if issue.IssueType == types.TypeAgent {
+		if issue.RoleType != "" {
+			label := "role_type:" + issue.RoleType
+			if err := store.AddLabel(ctx, issue.ID, label, s.reqActor(req)); err != nil {
+				return Response{
+					Success: false,
+					Error:   fmt.Sprintf("failed to add role_type label: %v", err),
+				}
+			}
+		}
+		if issue.Rig != "" {
+			label := "rig:" + issue.Rig
+			if err := store.AddLabel(ctx, issue.ID, label, s.reqActor(req)); err != nil {
+				return Response{
+					Success: false,
+					Error:   fmt.Sprintf("failed to add rig label: %v", err),
+				}
+			}
+		}
+	}
+
 	// Add dependencies if specified
 	for _, depSpec := range createArgs.Dependencies {
 		depSpec = strings.TrimSpace(depSpec)
@@ -482,6 +514,29 @@ func (s *Server) handleUpdate(req *Request) Response {
 		}
 	}
 
+	// Auto-add role_type/rig labels for agent beads when these fields are set
+	// This enables filtering queries like: bd list --type=agent --label=role_type:witness
+	if issue.IssueType == types.TypeAgent {
+		if updateArgs.RoleType != nil && *updateArgs.RoleType != "" {
+			label := "role_type:" + *updateArgs.RoleType
+			if err := store.AddLabel(ctx, updateArgs.ID, label, actor); err != nil {
+				return Response{
+					Success: false,
+					Error:   fmt.Sprintf("failed to add role_type label: %v", err),
+				}
+			}
+		}
+		if updateArgs.Rig != nil && *updateArgs.Rig != "" {
+			label := "rig:" + *updateArgs.Rig
+			if err := store.AddLabel(ctx, updateArgs.ID, label, actor); err != nil {
+				return Response{
+					Success: false,
+					Error:   fmt.Sprintf("failed to add rig label: %v", err),
+				}
+			}
+		}
+	}
+
 	// Handle reparenting
 	if updateArgs.Parent != nil {
 		newParentID := *updateArgs.Parent