ci: skillmd-lint baseline + report-only workflow

[Mine-FNL]

What this PR does

Two things:

1. Adds a report-only skillmd-lint CI workflow (findings surface as
   inline GitHub Actions annotations; the job does not fail).
2. Adds findings.md, a baseline audit of the current Skills/ tree:
   rule code, count, and file references, generated with
   skillmd-lint v1.4.0 --format json.

Why

79 SKILL.md files is a real maintenance surface. The baseline in
findings.md is:

• 130 errors: 28 files have no parseable YAML frontmatter
  (E002/E003/E005/E007), and 18 descriptions exceed the
  1024-char spec limit (E008).
• 295 warnings: missing ## Pitfalls to avoid / ## When to use
  sections, missing negative triggers, and bodies over 200 lines.

This overlaps with the frontmatter backfill work in #29 — the workflow
here is the CI half of that effort, so the baseline cannot regress
silently.

What changes

• .github/workflows/skillmd-lint.yml — report-only, no content
  changes, no merge blocking.
• findings.md — the baseline snapshot. Regenerate with
  skillmd-lint --format json $(find Skills -name SKILL.md).

Suggested follow-up

Once the frontmatter backfill lands, the workflow can drop
continue-on-error and gain --strict to become a real gate. Happy
to send that follow-up after #29 merges.

Maintenance

I maintain skillmd-lint and will keep the workflow current.

Reference

• skillmd-lint: https://github.com/Mine-FNL/skillmd-lint
• v1.4.0 release: https://github.com/Mine-FNL/skillmd-lint/releases/tag/v1.4.0
• 304 tests at 100% line + branch coverage, MIT