[codex] docs: clarify hooks notification example

[Randy-sin]

Summary

• Replace the non-functional Notification hook example that used permission_prompt
• Document that Notification matchers use notification types, not sink names
• Clarify that approval requests are not currently emitted as Notification hook events

Why

The documented matcher = "permission_prompt" example never fires because approval requests are handled through the approval runtime and wire request path, not through the notification manager.

This is a draft because #2335 already covers the core fix for #2330. The extra bit here is the explicit approval-request clarification and the use-case wording update.

Validation

• Parsed both updated TOML snippets with tomllib
• Ran bun run build in docs/