docs: trim skill READMEs and ADR/changelog narration

Two related simplification-audit findings, bundled because they edit
some of the same skill-audit files and splitting would fragment
single-file diffs.

Finding 10: delete 48 per-skill/reference README.md files (they
restated SKILL.md in narrative form and no agent ever loads them) plus
2 scaffold templates. Drop the README criterion from skill-audit's
file-structure.md and finding-criteria.md, and the README-generation
step from skill-author's new-skill.sh; update new-skill.bats to match.
Plugin-root READMEs are kept intentionally, out of scope.

Finding 12: strip historical ADR-0020/ADR-0023 citations and
changelog-style narration from model-facing skill content across
kyberforge and git plugin skills. Delete skill-author's one-time
retrofit.md migration guide and its references. Some ADR-0023 tags
were not narration but check-rtk-prefix's required opt-out marker for
intentionally-bare git commands -- those were restored, not stripped.

Mirror re-synced and full pre-commit/pre-push suite verified green.

Refs: SIMPLIFICATION-AUDIT.md findings 10, 12

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YR2CjVumUbEGWcMikcoXBD

plugins/kyberforge/skills/skill-audit/references/finding-criteria.md

@@ -111,13 +111,11 @@ Flag as FAIL if:
 - A path that resolves outside the skill directory appears outside the two exempt locations, in
   prose rather than in a fenced example
 - `tests/` exists but `tests/README.md` is missing or does not document its repo-level dependency
-- `README.md` is absent, or its file table has a missing or stale row
 - `SKILL.md` describes a script invocation the script does not accept
 
 Flag as SUGGESTION if:
 
 - An optional directory exists but holds only a placeholder README
-- `README.md` is accurate but describes a file's purpose more thinly than `SKILL.md` does
 
 ## formatting and scripts — `references/formatting-and-scripts.md`