feat: source control .blink/config.json file

Update .gitignore templates to allow .blink/config.json to be committed
while still ignoring other files in the .blink/ directory.

This enables teams to share agent configuration across the repository
while keeping build artifacts and temporary files out of source control.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: mattvollmer