feat: auto-register/unregister daemons via optional Registry

When Registry is set on DaemonOptions, Start() auto-registers the daemon
(filling PID and Health address) and Stop() auto-unregisters it. Consumers
without a registry are completely unaffected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

daemon.go

@@ -26,6 +26,13 @@ type DaemonOptions struct {
 
 	// HealthChecks are additional health check functions.
 	HealthChecks []HealthCheck
+
+	// Registry for tracking this daemon. Leave nil to skip registration.
+	Registry *Registry
+
+	// RegistryEntry provides the code and daemon name for registration.
+	// PID, Health, and Started are filled automatically.
+	RegistryEntry DaemonEntry
 }
 
 // Daemon manages daemon lifecycle: PID file, health server, graceful shutdown.
@@ -84,6 +91,19 @@ func (d *Daemon) Start() error {
 	}
 
 	d.running = true
+
+	// Auto-register if registry is set
+	if d.opts.Registry != nil {
+		entry := d.opts.RegistryEntry
+		entry.PID = os.Getpid()
+		if d.health != nil {
+			entry.Health = d.health.Addr()
+		}
+		if err := d.opts.Registry.Register(entry); err != nil {
+			return fmt.Errorf("registry: %w", err)
+		}
+	}
+
 	return nil
 }
 
@@ -128,6 +148,11 @@ func (d *Daemon) Stop() error {
 		}
 	}
 
+	// Auto-unregister
+	if d.opts.Registry != nil {
+		_ = d.opts.Registry.Unregister(d.opts.RegistryEntry.Code, d.opts.RegistryEntry.Daemon)
+	}
+
 	d.running = false
 
 	if len(errs) > 0 {

daemon_test.go

@@ -3,6 +3,7 @@ package process
 import (
 	"context"
 	"net/http"
+	"os"
 	"path/filepath"
 	"testing"
 	"time"
@@ -91,3 +92,33 @@ func TestDaemon_DefaultShutdownTimeout(t *testing.T) {
 	d := NewDaemon(DaemonOptions{})
 	assert.Equal(t, 30*time.Second, d.opts.ShutdownTimeout)
 }
+
+func TestDaemon_AutoRegisters(t *testing.T) {
+	dir := t.TempDir()
+	reg := NewRegistry(filepath.Join(dir, "daemons"))
+
+	d := NewDaemon(DaemonOptions{
+		HealthAddr: "127.0.0.1:0",
+		Registry:   reg,
+		RegistryEntry: DaemonEntry{
+			Code:   "test-app",
+			Daemon: "serve",
+		},
+	})
+
+	err := d.Start()
+	require.NoError(t, err)
+
+	// Should be registered
+	entry, ok := reg.Get("test-app", "serve")
+	require.True(t, ok)
+	assert.Equal(t, os.Getpid(), entry.PID)
+	assert.NotEmpty(t, entry.Health)
+
+	// Stop should unregister
+	err = d.Stop()
+	require.NoError(t, err)
+
+	_, ok = reg.Get("test-app", "serve")
+	assert.False(t, ok)
+}