Standardize daemon detection with tryDaemonLock probe (bd-wgu4)

- Extract lock checking to internal/lockfile package
- Add lock probe in RPC client before connection attempts
- Update daemon discovery to use lock probe
- Eliminates unnecessary connection attempts when socket missing

Closes bd-wgu4

Amp-Thread-ID: https://ampcode.com/threads/T-3b863f21-3af4-49d3-9214-477d904b80fe
Co-authored-by: Amp <amp@ampcode.com>

internal/daemon/discovery.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/steveyegge/beads/internal/lockfile"
 	"github.com/steveyegge/beads/internal/rpc"
 )
 
@@ -145,6 +146,25 @@ func discoverDaemon(socketPath string) DaemonInfo {
 		Alive:      false,
 	}
 
+	// Fast probe: check daemon lock before attempting RPC if socket doesn't exist
+	// This eliminates unnecessary connection attempts when no daemon is running
+	// If socket exists, we proceed with RPC for backwards compatibility
+	_, err := os.Stat(socketPath)
+	socketExists := err == nil
+	
+	if !socketExists {
+		beadsDir := filepath.Dir(socketPath)
+		running, _ := lockfile.TryDaemonLock(beadsDir)
+		if !running {
+			daemon.Error = "daemon lock not held and socket missing"
+			// Check for daemon-error file
+			if errMsg := checkDaemonErrorFile(socketPath); errMsg != "" {
+				daemon.Error = errMsg
+			}
+			return daemon
+		}
+	}
+
 	// Try to connect with short timeout
 	client, err := rpc.TryConnectWithTimeout(socketPath, 500*time.Millisecond)
 	if err != nil {