chore: prepare Artic v0.6.0 release

[baskduf]

Summary

• Bump Artic package, marketplace, plugin, skill, and generated metadata surfaces to 0.6.0.
• Add the 0.6.0 changelog entry for source-role metadata, runtime source-role gating, and reduced default source over-selection.
• Update localized README stable marketplace install pins to baskduf/artic@v0.6.0.
• Advance the stale-version regression guard from 0.4.1 to 0.5.0.

Release scope

• Release PR only; no tag or GitHub Release has been created.
• Intended tag after merge: v0.6.0.
• Confirmed v0.6.0 does not exist locally or remotely before opening this PR.

Test Plan

• python3 -m pytest -q → 104 passed
• python3 skills/artic/scripts/test_artic_scripts.py
• python3 plugins/claude-artic/skills/artic/scripts/test_artic_scripts.py
• python3 plugins/codex-artic/skills/artic/scripts/test_artic_scripts.py
• python3 -m compileall -q skills/artic/scripts plugins/claude-artic/skills/artic/scripts plugins/codex-artic/skills/artic/scripts
• python3 -m build --sdist --wheel
• python3 scripts/build_skill_archive.py --output dist/artic-skill-v0.6.0.tar.gz
• python3 scripts/check_release_artifacts.py --require-payload dist/artic-0.6.0.tar.gz dist/artic-skill-v0.6.0.tar.gz
• python3 scripts/check_release_artifacts.py dist/artic-0.6.0-py3-none-any.whl
• python3 skills/artic/scripts/artic_version.py --root . --no-network --json
• python3 skills/artic/scripts/artic_update.py --root . --no-network
• Producer smoke: artic_init.py writes brief.json/state.json with artic_version: 0.6.0 and preserves selected-source role metadata
• git diff --check
• Independent release PR review: no blockers