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_windows.go

@@ -0,0 +1,38 @@
+//go:build windows
+
+package lockfile
+
+import (
+	"errors"
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+var errDaemonLocked = errors.New("daemon lock already held by another process")
+
+// flockExclusive acquires an exclusive non-blocking lock on the file using LockFileEx
+func flockExclusive(f *os.File) error {
+	// LOCKFILE_EXCLUSIVE_LOCK (2) | LOCKFILE_FAIL_IMMEDIATELY (1) = 3
+	const flags = windows.LOCKFILE_EXCLUSIVE_LOCK | windows.LOCKFILE_FAIL_IMMEDIATELY
+
+	// Create overlapped structure for the entire file
+	ol := &windows.Overlapped{}
+
+	// Lock entire file (0xFFFFFFFF, 0xFFFFFFFF = maximum range)
+	err := windows.LockFileEx(
+		windows.Handle(f.Fd()),
+		flags,
+		0,             // reserved
+		0xFFFFFFFF,    // number of bytes to lock (low)
+		0xFFFFFFFF,    // number of bytes to lock (high)
+		ol,
+	)
+
+	if err == windows.ERROR_LOCK_VIOLATION || err == syscall.EWOULDBLOCK {
+		return errDaemonLocked
+	}
+
+	return err
+}