refactor(monorepo): simplify absorb/sync flows (-635 LoC)

- Drop the 17 per-package mirror CI workflows: they were inert in the monorepo
  and only re-tested, on the mirror, code the monorepo's own matrix already
  gated pre-merge. absorb now strips all incoming workflows and writes only
  mirror.yml; the ratchet lint over packages/* (no uses: left to pin) goes too.
- Make absorb's mirror ruleset idempotent: update whatever branch ruleset
  already exists in place instead of skipping on a name clash (the gap that left
  auth's mirror without the split-app bypass).
- Remove the standalone `import` command — unused, and it left a package without
  mirror.yml/banner/ruleset. import_package() stays as absorb's helper.
- Trim absorbing.md/creating.md/README to match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: loks0n