Extension onPostToolUse modifiedResult is not applied to the model's conversation context

## Summary

When a Copilot CLI extension returns `modifiedResult` from `onPostToolUse`, the modified text appears in the TUI tool-result preview line but is **not** applied to the model's conversation context. The model sees and responds with the original unmodified tool result.

## Reproduction

Minimal single-file extension (`.github/extensions/repro/extension.mjs`):

```js
import { joinSession } from "@github/copilot-sdk/extension";

joinSession({
tools: [
{
name: "say_hello",
description: "Returns a greeting message",
parameters: { type: "object", properties: {} },
async handler() {
return "ORIGINAL: The tool returned this text.";
},
},
],
hooks: {
async onPostToolUse(input) {
if (input.toolName !== "say_hello") return {};
return {
modifiedResult: {
textResultForLlm: "MODIFIED: The hook replaced the result with this text.",
resultType: "success",
},
};
},
},
});
```

### Steps

1. Place the above in a git repo under `.github/extensions/repro/extension.mjs`
2. Run `copilot --allow-all` from the repo root
3. Ask: "call say_hello and tell me exactly what it returned"

### Expected

The model reports: `MODIFIED: The hook replaced the result with this text.`

### Actual

The TUI tool-result preview correctly shows:
```
● say_hello
└ MODIFIED: The hook replaced the result with this text.
```

But the model responds with: `ORIGINAL: The tool returned this text.`

## Observations

- The hook **does** fire — stderr logging in the hook confirms execution.
- The TUI preview line shows the **modified** text.
- The model responds with the **original** text.
- This suggests the hook's `modifiedResult` is applied to the display layer but not to the model's message history.

## Impact

Any extension relying on `onPostToolUse` + `modifiedResult` for result transformation (sanitization, redaction, enrichment) is silently broken. The hook appears to work from the TUI, but the model operates on original data.

## Environment

- Copilot CLI 1.0.49-1
- Windows 11

Start with the reproduction in .github/extensions/repro/extension.mjs and trace onPostToolUse handling of modifiedResult from the hook through the tool-result preview and model conversation context. Verify where the original result enters message history. Done means the model reports the modified text while the TUI preview remains correct.

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