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.

[FooBarWidget]

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?

I don't know what "separate archive layout that mirrors live distros" means. Do you mean whether still-supported Ruby versions for still-supported distros should appear in the archive repo? The answer is no. The normal and archive repos should be non-intersecting sets. If the user wants to install either EOL'ed Ruby versions, or supported Ruby versions on EOL'd distros, then the user must add the archive repo. The normal repo is only for Ruby packages where both the Ruby version and the distro are still supported. If one of those two factors become EOL, then the corresponding packages should be archived.