fix: release standalone extensions with tags

[tractorjuice]

Summary

• make scripts/push-extensions.sh create or preserve vX.Y.Z tags and GitHub Releases for every standalone extension repo
• ensure the Gemini extension repo gets the gemini-cli-extension topic for gallery discovery
• remove pinned release numbers from extension READMEs and document that README release numbers are not allowed
• add release-process guardrails covering all six standalone extensions, version alignment, README drift, and publish-script coverage

Validation

• bash -n scripts/push-extensions.sh
• pytest tests/plugin/test_release_process.py
• pytest tests/codex/test_codex_extension.py
• pytest tests/vibe/test_vibe_extension.py tests/paperclip/test_commands_json.py
• pytest tests/plugin
• npx markdownlint-cli2 docs/RELEASING.md .claude/skills/release/SKILL.md extensions/arckit-codex/README.md extensions/arckit-vibe/README.md
• git diff --check