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