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>

.gitignore

@@ -1,3 +1,4 @@
 /plugins/
+!/plugins/
 !/plugins/cache-on-save/
 /data/

config/system.yaml

@@ -210,7 +210,7 @@ session:
   domain: null
   path: null
 gpm:
-  releases: stable
+  releases: testing
   official_gpm_only: true
 http:
   method: curl

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']