Feature Request: Add awaitingUserInput hook type

## Describe the feature or problem you'd like to solve

Currently, the `userPromptSubmitted` hook fires after the user submits input, but there's no hook that fires when the CLI is waiting for user input. This creates a gap for use cases that need to trigger actions when the agent is ready for interaction.

## Proposed solution

Add a new hook type: `awaitingUserInput` that fires when:
- The CLI is waiting for user input
- The agent has finished generating its response text
- Control is returned to the user

This would complement the existing hooks:
- `userPromptSubmitted` - fires after user hits enter
- **`awaitingUserInput`** (new) - fires when CLI awaits input

The hook should receive JSON input indicating the context:

```json
{
"inputType": "normal" | "ask_user",
"sessionId": "string",
"timestamp": "ISO8601"
}
```

This allows hooks to distinguish between:
- Normal conversation flow (after response generation completes)
- Interactive prompts (when `ask_user` tool displays choices/question)

**Benefits:**
- Enables audio/visual notifications when agent needs attention
- Improves accessibility for users relying on non-visual feedback
- Different handling for normal vs interactive (ask_user) inputs
- Session automation and logging capabilities

## Example prompts or workflows

### Workflow 1: Audio accessibility notifications
User sets up hook to play audio cues:
- Subtle sound when normal input is ready
- Urgent sound when `ask_user` requires immediate response
- Hook distinguishes between these using `inputType` field

### Workflow 2: Window focus management
User sets up hook to automatically focus terminal window when agent awaits input, particularly for `ask_user` interactions that require immediate decision.

### Workflow 3: Session metrics logging
User tracks time spent in different states:
- Response generation time (postToolUse → awaitingUserInput)
- User thinking time (awaitingUserInput → userPromptSubmitted)
- Interactive vs normal input patterns

### Workflow 4: IDE integration
IDE extension listens for `awaitingUserInput` hook to update status bar, show notifications, or adjust UI state when Copilot CLI needs attention.

### Workflow 5: Terminal bell notifications
Simple terminal bell when agent is idle and awaiting input, useful for multitasking users who want passive notifications.

## Example configuration

```json
{
"version": 1,
"hooks": {
"awaitingUserInput": [
{
"type": "command",
"bash": "./notify-ready.sh",
"cwd": "$HOME/.copilot/hooks",
"timeoutSec": 2
}
]
}
}
```

Example hook script:

```bash
#!/bin/bash
INPUT=$(cat)
INPUT_TYPE=$(echo "$INPUT" | jq -r '.inputType')

if [[ "$INPUT_TYPE" == "ask_user" ]]; then
# Play urgent sound for interactive questions
afplay /System/Library/Sounds/Sosumi.aiff &
else
# Play subtle sound for normal input
afplay /System/Library/Sounds/Ping.aiff &
fi
```

## Additional context

- GitHub Copilot CLI: v0.0.395
- OS: macOS
- Related issues: #1067 (user-level hooks), #1023 (prompt injection)
- Current workaround: None available - cannot detect when response generation completes

The gap this addresses:
1. Tool completes → sound plays (via `postToolUse` hook) ✅
2. Agent generates response text → no hook available ❌
3. Input awaited → no hook available ❌