Agent Host customizations view doesn't show plugin skills nested under category folders (`skills/<category>/<skill>/SKILL.md`)

- Copilot Chat Extension Version: 0.60.2026080401 (built-in)
- VS Code Version: 1.132.0-insider (x86_64)
- OS Version: macOS 15.7.7 (24G720)
- Feature (e.g. agent/edit/ask mode): Agent Host session (Copilot CLI) — Chat Customizations view
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): customendpoint/deepseek-v4-flash (model-agnostic; the bug is purely in view expansion)
- Logs: AHP logs under `~/Library/Application Support/Code - Insiders/logs//ahp/` show both plugins reporting `"load": {"kind": "loaded"}`. Can attach full logs on request.

Steps to Reproduce:

1. Add a local plugin directory via `chat.pluginLocations` pointing to a plugin using `skills///SKILL.md`, e.g. clone [mattpocock/skills](https://github.com/mattpocock/skills) and set `/path/to/mattpocock-skills` (its 22 skills live under `skills/engineering/*` and `skills/productivity/*`).
2. Open Chat → New Chat, choose session type **Agent Host (Copilot)** (requires `chat.agentHost.enabled: true`).
3. Open the **Customizations** view in the Chat panel.
4. The plugin entry `mattpocock-skills` is listed, but the skills tab is empty.
5. (Control) A plugin with single-level layout, e.g. [andrej-karpathy-skills](https://github.com/forrestchang/andrej-karpathy-skills) (`skills/karpathy-guidelines/SKILL.md`), shows its skill correctly.

Note: invoking skills via slash commands (`/mattpocock-skills:code-review`) works fine in the same session — the issue is confined to the Customizations view expansion.