test(checks): boundary cases for case-insensitive matching and Jaccard threshold

[protosphinx]

Why

Three behavioral invariants in runChecks were not covered by tests:

1. duplicate-name groups skills case-insensitively (Deploy and deploy are the same name).
2. description-collision fires at exactly the 0.6 Jaccard threshold (the boundary, >= 0.6).
3. name-drift lowercases both sides before comparing, so name: FOO correctly matches foo.md.

All three are real behaviors that a future refactor could silently break. Adding explicit regression tests locks them in.

What

• test/checks.test.ts: three new it blocks added to the existing runChecks describe suite.
  • duplicate-name match is case-insensitive
  • description-collision fires at exactly the 0.6 Jaccard threshold
  • name-drift match is case-insensitive

No source changes. No new dependencies.

Tests

• npm run lint && npm run build && npm test: all 92 tests pass (up from 89 before this PR).
• The Jaccard threshold case uses descriptions with token sets of size 4 sharing 3 tokens: 3 / (4 + 4 - 3) = 0.6 exactly.

Self-merge gate

• all CI checks pass
• LOC delta < 250 (added + removed): 26 lines added
• no public-API surface change
• no runtime-dependency additions
• no workflow file changes
• tests added or extended: yes, 3 new test cases

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