Replace the blanket cache invalidation (cleared on any file_write/edit)
with per-entry staleness detection based on file mtime and size. On cache
hit, stat the file and compare against the metadata stored at cache time.
If the file changed (user edited externally, git checkout, etc), the
stale entry is discarded and the tool re-executes.
This fixes the case where a user is coding alongside the agent — external
file changes are now detected instead of serving stale cached reads.
7e68c18303