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.

.geminiignore

@@ -77,6 +77,9 @@ __pycache__/
 # -----------------------------------------------------------------------------
 # Specific Directories
 # -----------------------------------------------------------------------------
+# Prototypes: We WANT Gemini to see these for context if they are synced locally,
+# even if they are ignored by Git. So we do NOT ignore them here.
+
 # Temporary migration folders
 _legacy/
 krow-workforce-export-latest/