Invalid example skill

This skill deliberately trips three rules so the CLI's diagnostic output can be inspected end-to-end. One fires under defaults; two need to be enabled to surface as errors (the spec ships them at lower severities).

1. invalid-skill-name (error by default) — the frontmatter name: is NotInvalid, which is not lowercase and does not match the parent directory invalid.
2. disallowed-field (disabled by default; enable via --disallowed-field or YAML config) — secret_field: is not in the spec's allowed field list.
3. check-absolute-paths (warning by default; escalate to error via --check-absolute-paths or YAML config) — the link below uses an absolute filesystem path, which is not portable across machines.

The broken link: absolute link

Run it with default rules:

dart run dart_skills_lint --skill ./example/skills/invalid

…and again with every rule turned up to error:

dart run dart_skills_lint --skill ./example/skills/invalid \
  --disallowed-field --check-absolute-paths

Expected: non-zero exit, error messages naming each rule that is enabled.