Add .vex.toml so `vex` is the project's primary code-search backend with
auto-update + semantic embeddings enabled. Ignore the .fastembed_cache/
directory that vex creates on first --semantic run. REVIEW_TODO.md
captures items flagged by the multi-agent production review that were
deliberately deferred (multi-day refactors, profile-first perf, and
design-sensitive security work).
