test: isolate community index cache from source tree

[guyua9]

Summary

• resolve the community index cache through an optional RULEBOOK_AI_INDEX_CACHE_PATH override
• route community-pack integration tests to temporary cache files
• preserve the bundled source cache fallback when the override is unset

Tests

• python3 -m compileall -q src/rulebook_ai/community_packs.py tests/integration/test_community_packs_index.py tests/integration/test_community_packs_list.py
• /opt/homebrew/bin/pytest -o addopts="" tests/integration/test_community_packs_list.py tests/integration/test_community_packs_index.py -q
• git diff --check

Result: 8 passed.