feat(oci/skills): add sentinel errors for packager failure classification

[samuv]

Summary

• Adds six exported sentinel errors in oci/skills (ErrInvalidSkillDir, ErrSkillMDMissing, ErrInvalidFrontmatter, ErrTooManyFiles, ErrSkillTooLarge, ErrInvalidSkillFile) so callers can classify packaging failures with errors.Is instead of brittle string matching
• Wraps every failure site in validateSkillDir, readSkillDirectory, parseFrontmatter, collectSkillFiles, and validateSkillFile with %w to the matching sentinel; original human-readable messages are preserved so existing string-based assertions and downstream UX strings keep working
• Scope is intentionally limited to packager errors — the tar.go / gzip.go / registry.go size and structure limits remain internal/transport concerns and can be promoted to sentinels later if a consumer needs to disambiguate them. No httperr mapping is added inside the package; HTTP semantics belong in the consumer's handler layer

Test plan

• task passes (lint + tests green, race detector enabled)
• TestPackager_Package_SentinelErrors: table-driven errors.Is coverage for missing/invalid dir, path traversal, missing SKILL.md, the four frontmatter failure modes (no opening delim, no closing delim, oversize, invalid YAML), missing name, and symlinked file/directory
• TestCollectSkillFiles_ExceedsMaxFiles extended with assert.ErrorIs(t, err, ErrTooManyFiles)
• TestCollectSkillFiles_ExceedsMaxSize: streams just over maxSkillTotalSize to disk and asserts ErrSkillTooLarge
• All pre-existing assert.Contains(t, err.Error(), ...) assertions kept untouched and still pass — message text is unchanged