Add stylistic lint checks to skill validate (#42)

[devrimcavusoglu]

Summary

• Add SeverityHint for non-blocking stylistic suggestions (distinct from errors/warnings)
• Add lintStyle() with 3 checks: body too short (<20 words), description too vague (<3 words), body lacks step-by-step structure
• Track hints separately in CLI output with ~ prefix and dedicated count
• HasErrors() correctly ignores hints

Test plan

• make lint && make test passes
• TestLintStyle_BodyTooShort, TestLintStyle_DescriptionTooVague, TestLintStyle_BodyLacksSteps, TestLintStyle_NoHints
• TestHasErrors includes "hints only" case
• TestValidate_ValidSkill updated with a well-formed body

Closes #42

🤖 Generated with Claude Code

[devrimcavusoglu]

Verified:

• make lint && make test — 0 lint issues, all packages pass
• 4 unit tests pass: TestLintStyle_BodyTooShort, _DescriptionTooVague, _BodyLacksSteps, _NoHints
• TestHasErrors "hints only" case confirms hints don't count as errors
• TestValidate_ValidSkill updated with 20+ word body — no issues
• Added 2 missing CLI-level tests in follow-up commit: TestSkillValidate_HintsJSON (verifies hints count in JSON, Valid=true) and TestSkillValidate_HintsText (verifies "hint(s)" summary and ~ prefix)