Pin Canopy checkout to decoupled_json branch

Canopy main still contains types/json which causes a json_idl_generate
target name collision with CanopyJSON's own interface. Use the
decoupled_json branch until the PR merges.

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

Author: edwardbr

.github/workflows/main.yml

@@ -23,11 +23,13 @@ jobs:
       # If Canopy is a private repository, add a personal access token as the
       # secret CANOPY_PAT (Settings → Secrets and variables → Actions) and
       # uncomment the token line below.
+      # TODO: remove ref once the decoupled_json PR is merged into Canopy main
       - name: Checkout Canopy
         uses: actions/checkout@v4
         with:
           repository: edwardbr/Canopy
           path: Canopy
+          ref: decoupled_json
           # token: ${{ secrets.CANOPY_PAT }}
 
       - name: Install dependencies