docs: document missing hooks and permission options in README

- Add 5 undocumented hooks: Startup Toast, Session Notification,
  Empty Task Response Detector, Grep/Tool Output Truncators
- Add Permission Options section with detailed table (edit, bash,
  webfetch, doom_loop, external_directory)
- Fix JSON schema: add 'build' to agents propertyNames

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

assets/oh-my-opencode.schema.json

@@ -65,6 +65,7 @@
       "propertyNames": {
         "type": "string",
         "enum": [
+          "build",
           "OmO",
           "oracle",
           "librarian",