No CI lane validates reference content under the four ignored artifact globs

Discovered while remediating review findings on #2599.

## Problem

`.markdownlint-cli2.jsonc` ignores four globs:

```
.github/instructions/**
.github/agents/**
.github/prompts/**
.github/skills/**
```

Every prompt-engineering artifact in the repository lives under one of them. `lint:md` reports zero issues on these paths without inspecting any of them.

`Validate-SkillStructure.ps1` runs on these paths but checks directory structure only, not file content. `lint:md-links` runs but is `soft-fail: true` in `pr-validation.yml`, so a green check on that lane carries no information.

The result: no lane inspects links, cross-artifact references, or the unavailable-reference clause on the artifacts that most depend on them.

## Evidence this is not theoretical

PR #2599 found ten broken relative script links in `hve-core-installer/SKILL.md` by manual inspection. CI passed the whole time.

The same PR then shipped eleven converted references without their required unavailable-reference prose. Nothing flagged it. A human reviewer found all twelve instances.

`mural/SKILL.md` was missing a trailing newline for its entire history.

## Suggested approach

`Validate-SkillStructure.ps1` already walks these paths and already has a `-ChangedFilesOnly` mode, so it is the natural host. Candidate checks:

- Relative links resolve from the containing file.
- No `.github/`-prefixed or absolute path appears inside a `SKILL.md`, which `vally-tests/references/skills.md` already documents as non-conforming but nothing enforces.
- A named cross-artifact reference is accompanied by unavailable-reference prose.

The third is the hardest to express mechanically and could start as a warning.

## Acceptance

- Broken relative links under the four globs fail a lane that is not soft-fail.
- The check runs on changed files in PR validation.
- The `vally-tests` non-conformance rule is enforced rather than only documented.