feat: add EOL package archival and pruning tooling

[abtreece]

Summary

• Add incremental APT/YUM archive migration scripts that move EOL distro packages to archive buckets
• Add APT/YUM pruning scripts that remove EOL Ruby version packages from still-supported distro repos
• Add comprehensive dev-handbook runbook for the archival process
• Update dev-handbook docs with accurate bucket names and archive architecture

Context

Addresses #190 — CI disk space exhaustion caused by the ever-growing Aptly state.tar.zst.

Two-axis cleanup:

1. Distro archival — Move entire EOL distro repos (centos-7, centos-8, debian-9, debian-10, ubuntu-18.04, ubuntu-20.04) to separate archive GCS buckets
2. Package pruning — Remove EOL Ruby version packages (2.5-3.1) from still-supported distro repos

Migration scripts are incremental — safe to re-run when future distros reach EOL. Each run fetches the existing archive state, merges new content, and uploads as version N+1.

Retiring centos-8 from the active build matrix (and any other EOL distro) is intentionally out of scope for this PR and will be handled as a follow-up that edits config.yml and regenerates workflows. This PR only delivers the tooling.

Companion PR: fullstaq-ruby/infra#47 (GCS buckets, DNS, Caddy config)
Implementation plan: context/plans/EOL-ARCHIVE-MIGRATION.md (Tasks 4-9)

New scripts

Script	Purpose
archive/migrate-apt-to-archive.rb	Move EOL distro APT repos to archive bucket
archive/migrate-yum-to-archive.rb	Move EOL distro YUM repos to archive bucket
archive/prune-apt-packages.rb	Remove EOL Ruby packages from APT state
archive/prune-yum-packages.rb	Remove EOL Ruby packages from YUM state

All scripts support --dry-run and auto-detect EOL content by comparing repo state against config.yml.

Test plan

• eclint check passes
• Migration scripts: --dry-run correctly identifies EOL distros/packages
• Migration scripts: incremental archive works (second run merges, doesn't overwrite)
• Pruning scripts: only fullstaq-ruby-X.Y* packages for non-active minor versions are removed
• Runbook procedures in dev-handbook/archiving-eol-packages.md match actual script behavior

[noahssarcastic]

Are we planning on announcing the removal of EOL Ruby packages in some way so that users can archive them themselves if they are still unmigrated?

[FooBarWidget]

Instead of pruning EOL packages, we should move them to the archive repos. EOL packages should still be installable.

[abtreece]

Agreed — installability of EOL Ruby packages on still-supported distros is a guarantee we shouldn't break. The current "pruning" approach is wrong for the same reason we archive EOL distros instead of deleting them.

I'll rework this:

• prune-apt-packages.rb / prune-yum-packages.rb → archive-move semantics (copy to archive bucket, then remove from live repo, idempotent, dry-run preserved). Rename to archive-* so the tooling name matches what it does.
• Update dev-handbook/archiving-eol-packages.md and the PR title/description to reflect archival, not pruning.
• Coordinate with fullstaq-ruby/infra#47 on bucket layout.

One clarifier before I start: should EOL-Ruby-on-supported-distro packages land in the existing per-distro archive buckets (extending their scope so each archive bucket can host EOL versions for both EOL and still-supported distros), or do you want a separate archive layout that mirrors live distros? The first is simpler; the second keeps "EOL distro archive" and "EOL version archive" as distinct concepts. Either way the answer shapes infra#47, so I'd like to settle it before reworking the scripts.

Also re: @noahssarcastic's question above — with archive-instead-of-delete, no user action is required to retain installability, but I'll still add a heads-up to release notes when this ships.