feat: add Grav 2.0 compat flag for cache-on-save plugin and switch GPM to testing channel

- Add blueprints.yaml for cache-on-save plugin with Grav 2.0 support
- Update system.yaml GPM setting from stable to testing channel
- Update .gitignore to allow cache-on-save plugin tracking

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/cache-on-save/blueprints.yaml

@@ -0,0 +1,13 @@
+name: Cache On Save
+version: 1.0.0
+description: Clears Grav cache on new-entry form submission
+author:
+  name: Mischa
+  email: mischa@gorinskat.nl
+license: MIT
+
+dependencies:
+  - { name: grav, version: '>=1.6.0' }
+
+grav:
+  version: ['1.7', '2.0']