feat(skills): add version pinning for agent skills (#314)

[Peolite001]

Summary

Adds explicit version tracking for agent skills. Skills can now be registered with a semver version, invoked with a pinned version, and listed with all available versions. Prevents silent breaking changes when a skill is updated.

Closes #314

Changes

• Register: POST /api/skills/register accepts optional version field (defaults to "1.0.0"). Duplicate id + version returns 409 Conflict.
• Invoke: POST /api/skills/:id/invoke?version=1.2.0 pins to exact version. Omitting version falls back to latest.
• List: GET /api/skills returns versions array per skill with all semver strings and latest pointer.
• Storage: Key changed from skillId to skillId@version. Latest pointer stored separately as latest:skillId.

API Examples

# Register v1.2.0
curl -X POST /api/skills/register \
  -d '{"skillId":"translate","version":"1.2.0","handler":"lib/skills/translate@v1.2.0"}'

# Invoke pinned version
curl -X POST '/api/skills/translate/invoke?version=1.2.0'

# Invoke latest (fallback)
curl -X POST /api/skills/translate/invoke

# List all skills with versions
curl /api/skills
# → { "skills": [{ "skillId": "translate", "versions": ["1.0.0","1.2.0"], "latest": "1.2.0" }] }

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud