Prune/consolidate archived experiment configs (configs/archive)

[apage224]

Summary

configs/ currently holds 206 YAMLs — 55 active plus 151 archived
under configs/archive/. The archive is exploratory experiment configs
accumulated during v1 development (sweeps, ablations, abandoned variants). They
are not part of the golden registry and are customer-visible.

Per AGENTS.md ("Customers will see this codebase — keep public-facing code,
configs, and docs clean"), the archive should be pruned or consolidated.

Context

• Found during the v1 self-review on PR v1 release prep: codec pipeline, goldens, robustness scorecards, and repo hygiene #42 (config sprawl flagged as
  pre-existing, out of that PR's scope).
• The golden registry (compressionkit/experiments/registry.py, 37 entries)
  references only active configs; nothing in configs/archive/ is wired into a
  registered golden.

Proposed work

• Confirm no registered golden / script / test references any
  configs/archive/*.yaml.
• Decide policy: delete outright, or move under the git-ignored internal/
  scratch area for historical reference.
• Document the retention policy in configs/README (or AGENTS.md) so the
  archive does not regrow.

Scope notes

• This is infrastructure/hygiene only — no behavior change to shipped goldens.
• Should be a standalone PR after v1 release prep: codec pipeline, goldens, robustness scorecards, and repo hygiene #42 merges.