refactor: focus Artic on init/start workflow

[baskduf]

Summary

• Remove the public @artic show / @artic review workflow surface.
• Remove show preview bundle generation, risk-readiness gating, production-readiness status fields, and related defensive output sections.
• Keep Artic focused on @artic init -> @artic start, sync Claude/Codex mirrors, and bump package/docs to 0.5.0.

Verification

• python3 -m py_compile skills/artic/scripts/*.py tests/*.py scripts/*.py
• python3 -m pytest -q -> 96 passed
• git diff --check
• Fresh 0.5.0 sdist/wheel build in /tmp/artic-build-check
• scripts/check_release_artifacts.py --require-payload /tmp/artic-build-check/artic-0.5.0.tar.gz
• scripts/check_release_artifacts.py /tmp/artic-build-check/artic-0.5.0-py3-none-any.whl
• Archive scan confirmed no artic_show.py, risk_readiness.py, 0.4.1, or v0.4.1 remnants

Notes

• Historical changelog entries for prior releases still mention removed features as history.
• Local ignored dist/ artifacts should not be reused for release; rebuild fresh from this branch/CI.