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.

Makefile

@@ -41,6 +41,7 @@ help:
 	@echo ""
 	@echo "  --- DEVELOPMENT TOOLS ---"
 	@echo "  make install-git-hooks - Installs git pre-push hook to protect main/dev branches."
+	@echo "  make sync-prototypes   - Builds and copies prototypes from adjacent 'client-krow-poc' repo."
 	@echo ""
 	@echo "  --- DATA CONNECT MANAGEMENT ---"
 	@echo "  make dataconnect-init         - Initializes Firebase Data Connect."