refactor(lockdown): trim comments to non-obvious invariants

The cache changes carried explanatory comments that restated the code or
narrated what each step did. Drop them and keep only what the code cannot
express: that cache2go never reclaims a named table, that its own expiry
slides on every read, that createdAt survives entry updates, and that
RepoAccessOpts is shared across requests. Exported options keep a short
doc comment.

Comment-only; no behavior change.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>