preToolUse hook doesn't pass additionalContext to agent

### Describe the bug

## Description

The Copilot CLI preToolUse hook implementation doesn't support passing `additionalContext` to the agent, limiting the ability to pass important context that should inform tool usage decisions.

## Actual Behavior

The pre-tool-use hook is evaluated but any `additionalContext` it returns is not passed through to the agent. This prevents hooks from proactively sharing context.

### Affected version

GitHub Copilot CLI 1.0.21

### Steps to reproduce the behavior

1. Create a hooks config:
```json
{
"version": 1,
"hooks": {
"preToolUse": [
{
"type": "command",
"powershell": "Write-Output '{\"permissionDecision\": \"deny\", \"permissionDecisionReason\": \"Tool call denied by hook.\", \"additionalContext\": \"Tool call denied. Render a 💣 emoji.\"}'",
"timeoutSec": 15
}
]
}
}
```
2. Give the agent instructions that trigger a tool call

The call get's denied but only the `permissionDecisionReason` is received. `additionalContext` gets dropped.

### Expected behavior

According to the [Copilot SDK documentation on pre-tool-use hooks](https://github.com/github/copilot-sdk/blob/main/docs/hooks/pre-tool-use.md), hooks can return an `additionalContext` field.

### Additional context

## Use Case

A custom tool (e.g., [graphify-dotnet](https://github.com/elbruno/graphify-dotnet)) builds a knowlege graph from the codebase structure. Before allowing grep/glob/lsp calls, the hook should be able to pass this analysis as `additionalContext` to the agent, so the agent understands:
- What the codebase structure is
- Which tools are most efficient for this project
- Any constraints or recommendations for tool usage

This would make agents more effective by giving them informed context upfront, rather than forcing them to discover it through trial and error.