feat: add bd agent state command for ZFC-compliant state reporting (bd-uxlb)

Add agent commands for self-reporting state:
- bd agent state <agent> <state>: Update agent state and last_activity
- bd agent heartbeat <agent>: Update last_activity timestamp only
- bd agent show <agent>: Display agent bead details

States: idle, spawning, running, working, stuck, done, stopped, dead

Also adds AgentState and LastActivity fields to UpdateArgs in RPC protocol.

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

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

internal/rpc/protocol.go

@@ -129,6 +129,9 @@ type UpdateArgs struct {
 	// Agent slot fields (gt-h5sza)
 	HookBead *string `json:"hook_bead,omitempty"` // Current work on agent's hook (0..1)
 	RoleBead *string `json:"role_bead,omitempty"` // Role definition bead for agent
+	// Agent state fields (bd-uxlb)
+	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
 }
 
 // CloseArgs represents arguments for the close operation