Feature request: a hook event for "agent is awaiting user input" (Notification-equivalent)

Goose's command-based hooks (`SessionStart`/`End`, `Stop`, `UserPromptSubmit`, `PreToolUse`/`PostToolUse`, …) make it easy for an external command to observe the agent. `Stop` gives turn-end, but there's no event that fires specifically when goose **pauses awaiting a human decision** — a tool-approval prompt or an interactive question (the "your turn" state). Claude Code exposes this as its `Notification` hook, and the Open Plugins hooks spec goose follows has room for it.

**Request:** add a `Notification`-style command hook that fires when goose blocks pending user input, with a payload noting the reason (e.g. `tool_permission`, `prompt`).

**Why:** external status tools — status bars and remote monitors — want a single reliable "waiting on you" signal rather than inferring it from `PreToolUse` (which fires on every tool, not on the block itself). Benefits any observer.

Look at the existing command-based hooks in the codebase, likely in a file like `src/hooks.rs` or `src/agent/events.rs`. Find where the agent transitions to a 'waiting for user input' state, such as after a tool permission request or an interactive prompt. Add a new hook variant and trigger it at that point, ensuring the payload includes the reason. Check test files for existing hook tests to understand the pattern and add a test for the new event.

索引モデルが issue の本文から書いたものです。