Add skill versioning and version management (#44)

[devrimcavusoglu]

Summary

• Add internal/skill/version.go with semver parsing, bumping (patch/minor/major), and comparison — stdlib only
• Add skern skill version <name> command to show current version
• Add skern skill version <name> --bump patch|minor|major to bump and persist
• Add --version flag to skern skill create for setting initial version (validates semver format)
• Add SkillVersionResult and VersionCompareResult output types for --json support

Test plan

• 35 unit tests for version parsing, string conversion, bump operations, and comparison (including edge cases)
• 14 CLI tests for show, bump (all levels), invalid input, sequential bumps, scoped resolution, create with version
• make build passes
• make test passes (all packages)
• make lint — 0 issues

Closes #44

🤖 Generated with Claude Code

[devrimcavusoglu]

Review Findings & Changes

Reviewed the PR comprehensively — overall well-structured with good test coverage and clean separation of concerns. Two issues identified and fixed in commit f090062:

1. Default skill version was 0.1.0, should be 0.0.1

The original code set new skills' default version to 0.1.0, which was confused with the CLI tool's own release version (v0.1.0). Skills should start at 0.0.1. Fixed in scaffold.go and updated the --version flag help text, AGENTS.md spec example, and all dependent test assertions.

2. Leading zeros accepted by ParseVersion

strconv.Atoi silently strips leading zeros, so "01.02.03" parsed as 1.2.3. Strict semver rejects leading zeros. Added a parseVersionPart helper that validates this, and 3 new test cases for coverage.

Not changed (confirmed correct)

• VersionCompareResult defined but unused — intentional forward scaffolding for upgrade detection
• Pre-release/build metadata not supported — correct for the SKILL.md spec's strict MAJOR.MINOR.PATCH format
• Bump level validated twice (CLI layer + BumpVersion) — intentional for proper exit code semantics

All checks pass locally: make lint (0 issues), make test (all packages), make test-smoke (51/51).