chore: ignore marvin major bumps in dependabot

Marvin upgrades can break the contact-extractor agent due to
pydantic-ai compatibility issues. Require manual testing before
accepting major version bumps.

Assisted-By: Claude (Anthropic AI) <noreply@anthropic.com>
Signed-off-by: Paolo Dettori <dettori@us.ibm.com>

Author: pdettori

.github/dependabot.yml

@@ -11,6 +11,10 @@ updates:
     directory: /a2a/a2a_contact_extractor
     schedule:
       interval: weekly
+    ignore:
+      - dependency-name: marvin
+        # marvin>=3.2.0 is pinned for pydantic-ai compatibility;
+        # major bumps need manual testing before merge
   - package-ecosystem: pip
     directory: /a2a/a2a_currency_converter
     schedule: