fix: resolve release-blocking Artic findings

[baskduf]

Summary

• Fixes release-blocking Artic lifecycle failures so invalid or missing strategy input no longer finalizes runtime/generated artifacts.
• Enforces strategy source_id grounding against the current .artic/references.json selected sources.
• Hardens skill archive/release artifact checks against unsafe paths and missing marketplace payload.
• Improves @artic init labeled-answer parsing and asset-policy messaging.
• Syncs canonical Artic skill changes to Claude/Codex plugin mirrors and fixes localized README drift.

Test Plan

• python3 -m pytest -q
• targeted lifecycle/source/show/sync pytest selection
• python3 -m py_compile skills/artic/scripts/*.py plugins/claude-artic/skills/artic/scripts/*.py plugins/codex-artic/skills/artic/scripts/*.py tests/*.py scripts/*.py
• python3 -m build --sdist --wheel --outdir /tmp/artic-release-check
• python3 scripts/build_skill_archive.py --output /tmp/artic-release-check/artic-skill-v0.3.0.tar.gz
• python3 scripts/check_release_artifacts.py --require-payload /tmp/artic-release-check/artic-0.3.0.tar.gz /tmp/artic-release-check/artic-skill-v0.3.0.tar.gz
• python3 scripts/check_release_artifacts.py /tmp/artic-release-check/artic-0.3.0-py3-none-any.whl
• plugin mirror byte comparison returned no diffs
• independent final review: P1/P2 findings gate clear