Define containment for project Agent source mutations

**Describe the bug**

Project-scoped Agent create and import operations use lexical project paths and ordinary filesystem writes. If a project-controlled Agent root or one of its ancestors is a link, creation can leave the selected project boundary.

This mutation surface is currently unused by the desktop client, so the product must decide whether to retain it before adding a substantial cross-platform filesystem containment subsystem.

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**To Reproduce**

1. Open a project whose Agent source root resolves through a link.
2. Invoke project-scoped Agent create or import through the ACP source API.
3. Observe that the filesystem operation follows the linked root.

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**Expected behavior**

Either remove the unused project Agent mutation surface, or define a containment contract that prevents any externally visible write outside the selected project, including during concurrent ancestor renames.

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**Screenshots**

Not applicable.

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**Please provide the following information**

- **OS & Arch:** Cross-platform filesystem behavior
- **Interface:** ACP source API
- **Version:** current `main`
- **Extensions enabled:** Not applicable
- **Provider & Model:** Not applicable

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**Additional context**

The design should choose one of these directions:

- remove project-scoped Agent create/import if the surface is not supported; or
- retain it with an explicit Unix symlink and Windows reparse-point/junction contract, root-anchored operations, and a concurrent-rename invariant.

The verification plan should cover linked roots and ancestors, create and import, collision naming, regular project behavior, Windows reparse points, and a race proving no out-of-project file becomes visible.

Do not begin implementation until this issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).

この issue は、root またはその祖先がシンボリックリンクである場合に、プロジェクトスコープの Agent ソース変更でセキュリティ境界に違反する可能性について説明しています。ACP source API がエントリーポイントです。Agent の create および import 操作の現在の実装を調査し、プロジェクトパスの解決方法とファイルシステムへの書き込み方法に注目してください。目標は、containment contract を定義するか、機能の削除を推奨することです。そのためには、クロスプラットフォームのファイルシステム動作と並行操作を深く理解する必要があります。

索引モデルが issue の本文から書いたものです。