fix: correct Copilot extension command registration (#1724)

* fix: correct Copilot extension command registration (#copilot)

- Use .agent.md extension for commands in .github/agents/
- Generate companion .prompt.md files in .github/prompts/
- Clean up .prompt.md files on extension removal
- Add tests for Copilot-specific registration behavior

Bumps version to 0.1.7.

* fix: test copilot prompt cleanup via ExtensionManager.remove() instead of manual unlink

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Co-authored-by: Ismael <ismael.jimenez-martinez@bmw.de>