fix(kyberforge): restore skill-audit's script-failure fallback and E100 diagnostic

The ADR-0020 body trim took `skill-audit` from 2,623 body words to a dispatch
shape, and two things went out with it that were not padding.

The manual structural fallback was one. Its replacement was a single sentence
telling the auditor to report an INFO when `validate.sh` cannot run — so with no
`python3` or no PyYAML, `skill-audit` reported the gap honestly and then audited
nothing structural at all. Every ADR-0020 measurement, the whole-file ceilings,
the name-to-directory match, the `references/` pointer check and the script
hygiene checks silently left the audit. A skill's whole Structure dimension
hanging on one optional interpreter is the same vacuous-pass shape the gate
scripts were just fixed for, one layer up.

The `E100 Runtime error ... does not exist` diagnostic was the other. That exit
code means an explicit relative `--config` was passed to `vale-wrap.sh` while
vale itself was installed and working; without the note, Step 1's fallback reads
exit 2 as "vale unavailable" and downgrades the description, body-discipline and
patterns dimensions to full LLM judgment for a config error it could have fixed.
That misreading is already recorded in CONTEXT.md as the reason both audit skills
stopped passing `--config` at all.

Both are restored in `references/validation-scripts.md`, loaded only when a Step 1
script fails — so the body pays nothing for them on a clean run, which is what the
dispatch pattern is for. The file also carries the by-hand boundary-target
procedure and the three ways to misread the result, including that
`INFO ... DID NOT RUN` is not a pass.

`references/file-structure.md` gains the one sanctioned spelling for a cross-skill
reference. The possessive form (``skill-audit's references/validation-scripts.md``)
is the only spelling both rules accept: a full repo path is what that section
already forbids, and a bare `references/<file>.md` is now a hard ERROR from the
ADR-0020 pointer check, which requires the file to exist in the skill's *own*
directory. Without the rule the two constraints look mutually exclusive.

Refs: ADR-0020

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

@@ -15,7 +15,7 @@
 - **URL:** https://agentskills.io/specification.md
 - **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
 - **Description:** Complete SKILL.md format specification — frontmatter fields, constraints, body content, optional directories, progressive disclosure levels, file references, validation
-- **Contributing files:** SKILL.md, references/body-discipline.md, references/description-quality.md, references/patterns.md, references/file-structure.md, references/formatting-and-scripts.md
+- **Contributing files:** SKILL.md, references/body-discipline.md, references/description-quality.md, references/patterns.md, references/file-structure.md, references/formatting-and-scripts.md, references/validation-scripts.md
 - **Status:** `extracted`
 
 ## agentskills-best-practices
@@ -47,7 +47,7 @@
 - **URL:** https://agentskills.io/skill-creation/using-scripts.md
 - **Research doc:** plugins/kyberforge/docs/research/docs/agentskillsio/sources.md
 - **Description:** Using scripts in skills — one-off commands, self-contained scripts with inline dependencies, designing scripts for agentic use (no interactive prompts, --help, structured output, idempotency)
-- **Contributing files:** SKILL.md, references/formatting-and-scripts.md
+- **Contributing files:** SKILL.md, references/formatting-and-scripts.md, references/validation-scripts.md
 - **Status:** `extracted`
 
 ## agentskills-quickstart