Simplify agent mode and re-add additional_permissions input

- Agent mode now only triggers when explicit prompt is provided
- Removed automatic triggering for workflow_dispatch/schedule without prompt
- Re-added additional_permissions input for requesting GitHub permissions
- Fixed TypeScript types for mock context helpers to properly handle partial inputs
- Updated documentation to reflect simplified mode behavior

src/modes/types.ts

@@ -26,7 +26,7 @@ export type ModeData = {
  *
  * Current modes include:
  * - 'tag': Interactive mode triggered by @claude mentions
- * - 'agent': General automation mode for all events (default)
+ * - 'agent': Direct automation mode triggered by explicit prompts
  */
 export type Mode = {
   name: ModeName;