simplify: remove workflow_dispatch/schedule from create-prompt

- Remove workflow_dispatch and schedule event handling from create-prompt
  since agent mode doesn't use the standard prompt generation flow
- Enforce mode compatibility at selection time in the registry instead
  of runtime validation in tag mode
- Add explanatory comment in agent mode about why prompt file is needed
- Update tests to reflect simplified event handling

This reduces code duplication and makes the separation between tag mode
(entity-based events) and agent mode (automation events) clearer.

src/modes/agent/index.ts

@@ -53,7 +53,10 @@ export const agentMode: Mode = {
       recursive: true,
     });
 
-    // Write the prompt - either override_prompt, direct_prompt, or a minimal default
+    // Write the prompt file - the base action requires a prompt_file parameter,
+    // so we must create this file even though agent mode typically uses
+    // override_prompt or direct_prompt. If neither is provided, we write
+    // a minimal prompt with just the repository information.
     const promptContent =
       context.inputs.overridePrompt ||
       context.inputs.directPrompt ||