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/server_issues_epics.go

@@ -108,6 +108,13 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.RoleBead != nil {
 		u["role_bead"] = *a.RoleBead
 	}
+	// Agent state fields (bd-uxlb)
+	if a.AgentState != nil {
+		u["agent_state"] = *a.AgentState
+	}
+	if a.LastActivity != nil && *a.LastActivity {
+		u["last_activity"] = time.Now()
+	}
 	return u
 }