Make cache .gitignore updates configurable

[willhains]

Summary

• Check Git's complete ignore configuration with git check-ignore before changing a repository's .gitignore.
• Add copilot-specs.cacheGitignoreBehavior with auto, prompt, and disabled modes.
• Remember prompt decisions separately for each workspace folder.
• Apply the behaviour correctly in multi-root workspaces.

Why

The extension currently attempts to add .copilot-specs-cache/ during activation.
The helper only inspects the repository's own .gitignore, so it modifies a tracked project file even when Git already ignores the cache through core.excludesFile or another supported source.
Users also have no way to opt out or ask the extension to prompt first.

User impact

The default remains auto for compatibility, but the extension now leaves .gitignore unchanged when Git already ignores the cache.
Users can choose prompt to approve the change per workspace folder, or disabled to manage ignore rules themselves.

Validation

• npm run compile
• npm test -- --disable-extensions - 58 passing
• npm run lint could not run because the repository does not install an eslint executable; the existing script exits with eslint: command not found.