What does this PR do?

Prevents the C test runner from reading or writing a user's real codebase-memory cache.

Several integration fixtures exercise the production index_repository flow. The production cache resolver prefers CBM_CACHE_DIR, while older fixtures derive the expected database path from HOME/.cache/codebase-memory-mcp. When the runner inherited a real cache, early assertion returns could leave fixture databases behind. Setting only CBM_CACHE_DIR was also insufficient because those legacy paths diverged.

Fix

• create a private temporary HOME for every normal test-runner process;
• clear inherited CBM_CACHE_DIR, so production code and legacy fixtures converge on the private HOME cache;
• delete the sentinel tree with atexit, including after early test returns;
• make the lang-contract helper honor an explicit CBM_CACHE_DIR when a test intentionally sets one.

Worker/probe subprocess modes still exit before the sentinel setup.

Validation

Windows GCC build after rebasing onto current main:

• full C runner: 5831 passed, 22 skipped;
• production-cache sentinel: 501 DB files before, 501 after;
• added files: 0;
• removed files: 0.

The same run previously left cbm_seq898_*, cbm_lc_*, and cbm_fcw_* fixture databases in the inherited cache.

Checklist

• Every commit is signed off (git commit -s)
• Full C test runner passes locally
• Full lint (left to CI)
• Behavior validated with a before/after cache sentinel