test: add full extension suites

[tractorjuice]

Summary

• add generated-output test suites for Gemini, OpenCode, and Copilot extensions
• add shared extension test helpers so all target suites use the converter's full command and agent source surface
• fix non-Claude converter rewrites for user config placeholders and Copilot slash-command syntax
• add the full extension matrix to release docs and the release skill checklist

Validation

• python scripts/converter.py
• pytest tests/gemini tests/opencode tests/copilot
• pytest tests/codex/test_codex_extension.py tests/vibe/test_vibe_extension.py tests/paperclip/test_commands_json.py tests/plugin/test_release_process.py
• pytest tests
• npx markdownlint-cli2 docs/RELEASING.md .claude/skills/release/SKILL.md
• git diff --check