Preserve manual .gitattributes entries after genkit regeneration (#4667)

Add .gitattributes.manual for hand-maintained entries (currently the
merge=union strategy for NEXT_CHANGELOG.md) and update the generate
target to prepend it to the genkit-produced .gitattributes.

I have added manual line #4639 but
it's overwritten each time "make generate" is ran.

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: denik

.gitattributes

@@ -1,4 +1,6 @@
 NEXT_CHANGELOG.md merge=union
+
+# Generated by genkit update-sdk:
 cmd/account/access-control/access-control.go linguist-generated=true
 cmd/account/billable-usage/billable-usage.go linguist-generated=true
 cmd/account/budget-policy/budget-policy.go linguist-generated=true

.gitattributes.manual

@@ -0,0 +1,3 @@
+NEXT_CHANGELOG.md merge=union
+
+# Generated by genkit update-sdk:

Makefile

@@ -181,6 +181,7 @@ generate:
 	cd $(UNIVERSE_DIR) && bazel build //openapi/genkit
 	@echo "Generating CLI code..."
 	$(GENKIT_BINARY) update-sdk
+	cat .gitattributes.manual .gitattributes > .gitattributes.tmp && mv .gitattributes.tmp .gitattributes
 	@echo "Updating direct engine config..."
 	make generate-direct