[codex] Fix Codex stop hook index fallback

[stevenkleinhens]

Summary

• Fix the Codex stop hook worker temp-file template so macOS mktemp creates unique files instead of a literal XXXXXX.json path.
• Avoid full stop-hook reindexing when a watch process is already alive.
• Add a one-at-a-time background index fallback for cases where no watch is running.

Root cause

The stop hook used a macOS-incompatible mktemp ...XXXXXX.json template and then ran a full memsearch index after each stop-hook memory write. With repeated stop workers, that could overwrite worker payloads and spawn redundant full index processes even though memsearch watch should normally own incremental indexing.

Validation

• uv run pytest tests/test_codex_stop_hook_worker.py tests/test_codex_stop_hook_utf8.py -q
• uv run ruff check tests/test_codex_stop_hook_worker.py
• bash -n plugins/codex/hooks/common.sh plugins/codex/hooks/stop.sh
• git diff --check

Note: a full uv run pytest -q reached 199 passed, 7 skipped before I interrupted a Milvus Lite reopen test that was retrying a refused local socket; that appears unrelated to this hook change.