Validate skill bash snippets in tests

[xuezhaolan]

Summary

• add a SKILL.md freshness test that runs bash -n over concrete bash/sh fenced snippets
• skip placeholder/example snippets such as <url>, {{PLACEHOLDER}}, and ellipsis blocks
• batch syntax checks per markdown file for speed, then fall back to per-block checks for actionable file/line errors

Fixes #1667.

Test plan

• bun test test/skill-parser.test.ts test/skill-validation.test.ts

Note: I also tried bun test; it did not complete cleanly on this checkout due existing unrelated failures/timeouts, including a missing .factory/skills/gstack-ship/SKILL.md golden fixture and a timeout in browse/test/security-audit-r2.test.ts.