`github.copilot.chat.summarizeAgentConversationHistory.enabled` not recognized in per-directory workspace settings

**Summary:** A per-directory workspace is not recognizing the `github.copilot.chat.summarizeAgentConversationHistory.enabled` setting, even though it recognizes it in a separate multi-root workspace.

I have the following settings on a per-workspace basis:

```json
"chat.disableAIFeatures": false,
"github.copilot.chat.summarizeAgentConversationHistory.enabled": false
```

The first enables GitHub Copilot on a per-workspace basis. The second _disables automatic summarization_.

**Disabling automatic summarization / compaction is crucial.** After many months and tickets of inadequate and erronous summarization, I am finally able to make progress by turning off summarization altogether and summarizing only when I need to.

This has been working just fine, especially in my many-workspace-root custom workspaces (i.e. `*.code-workspace`).

* However I just went to a per-directory workspace (i.e. using `.vscode/settings.json` in the directory), and I pulled up the `settings.json` from within VS Code and it showed `github.copilot.chat.summarizeAgentConversationHistory.enabled` grayed out, and said it wasn't recognized.
* Going into the actual settings UI, "Summarize Agent Context History" does not appear.
* However in the `*.code-workspace` settings, "Summarize Agent Context History" appears in the settings UI, and `github.copilot.chat.summarizeAgentConversationHistory.enabled` is not grayed out in the settings JSON editor.

What happened? Did a new version of VS Code GitHub Copilot accidentally stop recognizing `github.copilot.chat.summarizeAgentConversationHistory.enabled` in per-directory workspaces, or did I do something incorrectly to make that setting not get recognized (although I don't know what it would be)?

```
Version: 1.135.0 (user setup)
Commit: 08d4889f9ec4a1685d257b9b95de036c8e1ce1e5
Date: 2026-08-25T14:26:52Z
Electron: 42.8.1
ElectronBuildId: 14906494
Chromium: 148.0.7778.280
Node.js: 24.18.1
V8: 14.8.178.38-electron.0
@github/copilot: 1.0.81-0
@github/copilot-sdk: 1.0.11
OS: Windows_NT x64 10.0.26200
```