The weekly audit matched the existing "### <arch>" issue subsection by its
bare header ("### arm64") while writing headers with a
" (N missing, M to rebuild)" suffix. The equality test never matched, so every
run appended a fresh block instead of replacing the old one. Issue bodies
accumulated dozens of stale subsections (alpine-323 had 38 where 2 are
expected), and fetch-rebuild-packages-from-issue.R read the *first* block,
feeding the rebuild a months-old list while the genuinely-missing packages were
never rebuilt.
- Match arch subsections by prefix and drop all blocks for the arch before
writing one fresh block, so accumulation self-heals each run.
- Add local/dedupe-audit-issue.R to collapse the already-accumulated
duplicates in the existing issues (keeps the freshest block per arch).
- Make fetch-rebuild-packages-from-issue.R prefer the audit's freshly-written
RDS, falling back to issue parsing when it is absent.
