feat(checks): add empty-body rule

[protosphinx]

Why

A skill file with valid frontmatter but no body gives Claude nothing to act on. The skill will be selected based on its description, then fail silently because there are no instructions to execute. This closes a gap in the rule set: every other authoring mistake (bad schema, wrong tool refs, trigger collisions) is caught, but a completely empty body slipped through.

What

• src/checks.ts: adds checkEmptyBody(v), called in the main runChecks loop after schema validation. Fires warn / empty-body when v.body.trim().length === 0. Runs only on validated skills, so a schema-broken skill does not also get an empty-body warning.
• src/sarif.ts: registers empty-body in the static RULES catalog so GitHub Code Scanning shows a stable description and severity rather than falling back to the synthetic entry.
• test/checks.test.ts: four new cases covering empty string body, whitespace-only body, body with content (no warning), and invalid frontmatter with empty body (no warning).

Tests

• warns when skill body is empty: passes "" as body, expects empty-body diagnostic.
• warns when skill body is whitespace only: passes " \n\n " as body, expects empty-body diagnostic.
• does not warn on empty-body when body has content: verifies no false positive for a normal skill.
• does not fire empty-body when frontmatter is invalid: verifies the check is skipped when schema validation failed (the loop already continues past such skills).

Self-merge gate

• all CI checks pass
• LOC delta < 250 (added + removed)
• no public-API surface change
• no runtime-dependency additions
• no workflow file changes
• tests added or extended

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Generated by Claude Code