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.
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