chore: prepare Artic v0.4.0 release

[baskduf]

Summary

• Bumps Artic to 0.4.0 across package, marketplace, plugin, and skill version surfaces.
• Adds the 0.4.0 changelog entry for the asset-first show bundle and risk-readiness gate work from PRs feat: add asset-first Artic show preview bundle #27/feat: add Artic risk readiness gate #28.
• Updates stable localized README install pins to baskduf/artic@v0.4.0.
• Tightens the mirrored skill sync regression now that Claude/Codex payloads include the risk-readiness/show artifacts.

Verification

• python3 skills/artic/scripts/artic_version.py --root . --no-network --json
• mirror byte-for-byte check for canonical, Claude, and Codex skill payloads
• python3 -m py_compile skills/artic/scripts/*.py plugins/claude-artic/skills/artic/scripts/*.py plugins/codex-artic/skills/artic/scripts/*.py
• pytest → 130 passed
• python3 -m build
• python3 scripts/build_skill_archive.py --output dist/artic-skill-v0.4.0.tar.gz
• shasum -a 256 -c dist/SHA256SUMS.txt
• python3 scripts/check_release_artifacts.py --require-payload dist/artic-0.4.0.tar.gz dist/artic-skill-v0.4.0.tar.gz
• python3 scripts/check_release_artifacts.py dist/artic-0.4.0-py3-none-any.whl

Release note

Do not tag until this release PR is merged. Intended tag after merge: v0.4.0.