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, 23 insertions, 0 deletions

diff --git a/internal/filelock/lock_posix.go b/internal/filelock/lock_posix.go
new file mode 100644
index 0000000..5e49e3b
--- /dev/null
+++ b/internal/filelock/lock_posix.go
@@ -0,0 +1,23 @@
+//go:build !windows
+
+package filelock
+
+import (
+	"errors"
+
+	"golang.org/x/sys/unix"
+)
+
+func tryLockExclusive(fd uintptr) error {
+	if err := unix.Flock(int(fd), unix.LOCK_EX|unix.LOCK_NB); err != nil {
+		if errors.Is(err, unix.EWOULDBLOCK) || errors.Is(err, unix.EAGAIN) {
+			return ErrWouldBlock
+		}
+		return err
+	}
+	return nil
+}
+
+func unlockExclusive(fd uintptr) error {
+	return unix.Flock(int(fd), unix.LOCK_UN)
+}