feat: support configuring a default_agent across all API/user surfaces (#5843)

Co-authored-by: observerw <observerw@users.noreply.github.com>

github/action.yml

@@ -9,6 +9,10 @@ inputs:
     description: "Model to use"
     required: true
 
+  agent:
+    description: "Agent to use. Must be a primary agent. Falls back to default_agent from config or 'build' if not found."
+    required: false
+
   share:
     description: "Share the opencode session (defaults to true for public repos)"
     required: false
@@ -62,6 +66,7 @@ runs:
       run: opencode github run
       env:
         MODEL: ${{ inputs.model }}
+        AGENT: ${{ inputs.agent }}
         SHARE: ${{ inputs.share }}
         PROMPT: ${{ inputs.prompt }}
         USE_GITHUB_TOKEN: ${{ inputs.use_github_token }}