ask: serialize concurrent CLI with repo lock and stale PID recovery

Add advisory lock under .git/hexai-ask.lock around Taskwarrior execution,
with metadata (PID and process basename) and Linux /proc comm checks to
remove orphan lock files when the recorded holder is gone or not ask.

Extract internal/filelock for shared flock helpers; stats uses it too.

Made-with: Cursor

1 files changed, 24 insertions, 0 deletions

diff --git a/internal/filelock/lock_windows.go b/internal/filelock/lock_windows.go
new file mode 100644
index 0000000..058b942
--- /dev/null
+++ b/internal/filelock/lock_windows.go
@@ -0,0 +1,24 @@
+//go:build windows
+
+package filelock
+
+import (
+	"golang.org/x/sys/windows"
+)
+
+func tryLockExclusive(fd uintptr) error {
+	var ol windows.Overlapped
+	err := windows.LockFileEx(windows.Handle(fd), windows.LOCKFILE_EXCLUSIVE_LOCK|windows.LOCKFILE_FAIL_IMMEDIATELY, 0, 1, 0, &ol)
+	if err == nil {
+		return nil
+	}
+	if err == windows.ERROR_LOCK_VIOLATION {
+		return ErrWouldBlock
+	}
+	return err
+}
+
+func unlockExclusive(fd uintptr) error {
+	var ol windows.Overlapped
+	return windows.UnlockFileEx(windows.Handle(fd), 0, 1, 0, &ol)
+}