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>

cmd/bd/agent_test.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestValidAgentStates(t *testing.T) {
+	// Test that all expected states are valid
+	expectedStates := []string{
+		"idle", "spawning", "running", "working",
+		"stuck", "done", "stopped", "dead",
+	}
+
+	for _, state := range expectedStates {
+		if !validAgentStates[state] {
+			t.Errorf("expected state %q to be valid, but it's not", state)
+		}
+	}
+}
+
+func TestInvalidAgentStates(t *testing.T) {
+	// Test that invalid states are rejected
+	invalidStates := []string{
+		"starting", "waiting", "active", "inactive",
+		"unknown", "error", "RUNNING", "Idle",
+	}
+
+	for _, state := range invalidStates {
+		if validAgentStates[state] {
+			t.Errorf("expected state %q to be invalid, but it's valid", state)
+		}
+	}
+}
+
+func TestAgentStateCount(t *testing.T) {
+	// Verify we have exactly 8 valid states
+	expectedCount := 8
+	actualCount := len(validAgentStates)
+	if actualCount != expectedCount {
+		t.Errorf("expected %d valid states, got %d", expectedCount, actualCount)
+	}
+}
+
+func TestFormatTimeOrNil(t *testing.T) {
+	// Test nil case
+	result := formatTimeOrNil(nil)
+	if result != nil {
+		t.Errorf("expected nil for nil time, got %v", result)
+	}
+}