askcli: serialize task alias cache writes to fix concurrent rename race

Two concurrent 'ask' invocations could race on the alias cache file:
both wrote the JSON to a shared '<path>.tmp' filename and then both
called os.Rename, so the loser failed with:

  replace task alias cache: rename .../task-aliases-v2.json.tmp
    .../task-aliases-v2.json: no such file or directory

The shared tempfile also enabled lost updates because each process
loaded the file independently before saving its own version on top.

Fix:
- Take an exclusive flock on a sentinel file (task-aliases-v2.json.lock)
  in the cache directory around the full load/modify/save cycle in both
  ensureTaskAliases and resolveTaskSelectorFromCache, using the existing
  internal/filelock package.
- Switch save() to os.CreateTemp so each writer gets a unique tempfile
  name; the loser's tempfile is removed cleanly on rename failure.
- Refactor resolveTaskSelectorFromCache by extracting
  finalizeResolvedTaskSelector to keep functions under 50 lines.

Adds TestEnsureTaskAliases_ConcurrentCallsDoNotRaceOnTempFile, which
reproduces the original error reliably on the unfixed code and now
passes with -race.

Amp-Thread-ID: https://ampcode.com/threads/T-019df49f-52a5-75b1-98d5-371a163ef100
Co-authored-by: Amp <amp@ampcode.com>

0 files changed, 0 insertions, 0 deletions