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/lockfile/lock_unix.go

@@ -0,0 +1,21 @@
+//go:build unix
+
+package lockfile
+
+import (
+	"errors"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+var errDaemonLocked = errors.New("daemon lock already held by another process")
+
+// flockExclusive acquires an exclusive non-blocking lock on the file
+func flockExclusive(f *os.File) error {
+	err := unix.Flock(int(f.Fd()), unix.LOCK_EX|unix.LOCK_NB)
+	if err == unix.EWOULDBLOCK {
+		return errDaemonLocked
+	}
+	return err
+}