Update docs for v1.3.0, remove stale planning artifacts

- Add v1.3.0 changelog entry for Python and Rust SkillSigner
- Bump technical specification to v1.3 with Section 16 (skill
  folder signing: canonicalization, .schemapin.sig format,
  verification flow, tamper detection)
- Update roadmap: Python and Rust SkillSigner shipped
- Remove GO_IMPLEMENTATION_SPECIFICATION.md (superseded by impl)
- Remove SCHEMAPIN_ENHANCEMENT_PLAN.md (all phases completed)
- Remove IMPLEMENTATION_PLAN.md (superseded by codebase)

Author: jaschadub