feat(kyberforge): give skill-author a procedure for the #99 retrofit

ADR-0020 shipped its gates hot with no baseline file, so 26 of 39 descriptions
and 9 of 39 bodies are over their FAIL tier and editing any of them for any
reason requires bringing the skill into contract first. `references/improve.md`
said exactly that and stopped there — it mandated a retrofit and supplied no
procedure for one.

Four dry-run retrofits confirmed what that costs. Asked the same questions —
what to cut first, when a body is two flows rather than one, what else has to
change alongside — they invented six to ten different answers, so the same skill
retrofitted twice produced two different skills and neither run could be reviewed
against anything.

`references/retrofit.md` fixes the answers: an ordered cut list ranked by tokens
removed against behaviour lost (inverting that order is how a retrofit deletes the
instruction the skill existed to carry), the test for whether a body holds two
mutually exclusive flows, the reference-file conventions, the collateral checklist
for `README.md` and `references/sources.md`, and a worked description retrofit.

It also states the trap the dry runs kept hitting: retrofit the skill in place,
inside its package. The boundary-target universe is built by walking up from the
file being checked, so a scratch copy has no authoring root above it, the check
prints `INFO ... DID NOT RUN`, and the run still exits 0 — a line that reads as a
pass and is not one. A retrofit signed off on a copy carries an unverified
boundary target into the corpus.

Loaded from the improve flow only when a budget is actually exceeded, so a routine
improvement pays nothing for it.

Refs: ADR-0020, #99

plugins/kyberforge/skills/skill-author/references/sources.md

@@ -34,7 +34,7 @@ source_keys:
 - **URL:** https://agentskills.io/skill-creation/best-practices.md
 - **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
 - **Description:** Best practices for skill creators — starting from real expertise, spending context wisely, calibrating control, instruction patterns (gotchas, templates, checklists, validation loops)
-- **Contributing files:** SKILL.md, references/create.md, references/improve.md, references/contract.md
+- **Contributing files:** SKILL.md, references/create.md, references/improve.md, references/contract.md, references/retrofit.md
 - **Status:** `extracted`
 
 ## agentskills-optimizing-descriptions
@@ -42,7 +42,7 @@ source_keys:
 - **URL:** https://agentskills.io/skill-creation/optimizing-descriptions.md
 - **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
 - **Description:** How to systematically test and improve skill descriptions for triggering accuracy — eval queries, trigger rate testing, train/validation splits, optimization loop
-- **Contributing files:** SKILL.md, references/improve.md, references/contract.md
+- **Contributing files:** SKILL.md, references/improve.md, references/contract.md, references/retrofit.md
 - **Status:** `extracted`
 
 ## agentskills-evaluating-skills