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/create-prompt/index.ts

@@ -350,24 +350,6 @@ export function prepareContext(
       };
       break;
 
-    case "workflow_dispatch":
-      eventData = {
-        eventName: "workflow_dispatch",
-        isPR: false,
-        ...(baseBranch && { baseBranch }),
-        ...(claudeBranch && { claudeBranch }),
-      };
-      break;
-
-    case "schedule":
-      eventData = {
-        eventName: "schedule",
-        isPR: false,
-        ...(baseBranch && { baseBranch }),
-        ...(claudeBranch && { claudeBranch }),
-      };
-      break;
-
     default:
       throw new Error(`Unsupported event type: ${eventName}`);
   }
@@ -430,18 +412,6 @@ export function getEventTypeAndContext(envVars: PreparedContext): {
           : `pull request event`,
       };
 
-    case "workflow_dispatch":
-      return {
-        eventType: "WORKFLOW_DISPATCH",
-        triggerContext: `workflow dispatch event`,
-      };
-
-    case "schedule":
-      return {
-        eventType: "SCHEDULE",
-        triggerContext: `scheduled automation event`,
-      };
-
     default:
       throw new Error(`Unexpected event type`);
   }