feat: add prototype syncing for local dev and AI context

This commit introduces a new feature to synchronize prototypes
from an external repository for local development and to provide
context for AI tools.

- Added `make sync-prototypes` target to Makefile to build and
 copy prototypes from the adjacent 'client-krow-poc' repository.
- Added `.geminiignore` entries to prevent Gemini from ignoring
 prototypes, ensuring they are available for context.
- Added `.gitignore` entries to ignore generated prototype files,
 except for the `.keep` files, to keep the directories.
- Added `04-sync-prototypes.md` documentation to explain how to
 sync prototypes.
- Removed empty `internal/prototypes/.keep` file.

README.md

@@ -62,6 +62,7 @@ This project uses a modular `Makefile` for all common tasks.
 -   **[01-backend-api-specification.md](./docs/01-backend-api-specification.md)**: (Legacy) Reference for data schemas.
 -   **[02-codemagic-env-vars.md](./docs/02-codemagic-env-vars.md)**: Guide for CI/CD environment variables.
 -   **[03-contributing.md](./docs/03-contributing.md)**: Guidelines for new developers and setup checklist.
+-   **[04-sync-prototypes.md](./docs/04-sync-prototypes.md)**: How to sync prototypes for local dev and AI context.
 
 ## 🤝 Contributing
 New to the team? Please read our **[Contributing Guide](./docs/03-contributing.md)** to get your environment set up and understand our workflow.