feat: add Artic risk readiness gate

[baskduf]

Summary

• Adds a risk/quality readiness contract for Artic so implementation readiness reflects project-specific risks, not only the four baseline init fields.
• Promotes dynamic follow-up requirements for asset, interaction, brand, runtime, license, accessibility/performance, and conversion dependencies.
• Propagates risk summaries, placeholder/fallback boundaries, substitute stop rules, and intent-matched acceptance criteria through init/start/show validation.
• Syncs canonical Artic changes into Claude and Codex plugin mirrors.

Test Plan

• python3 -m pytest -q tests/test_artic_package.py::test_skill_copies_are_in_sync
• python3 -m py_compile skills/artic/scripts/*.py plugins/claude-artic/skills/artic/scripts/*.py plugins/codex-artic/skills/artic/scripts/*.py
• python3 -m unittest discover -s skills/artic/scripts -v
• python3 -m pytest -q → 124 passed

Review

• Static scan: no added-line matches for hardcoded secret, shell=True/os.system, eval/exec, pickle.loads, or SQL string-format patterns.
• Independent findings review: initial blockers fixed; final review returned no blocking findings and no suggestions.