Plugin skills not included in available_skills for main agent — only builtin skills visible

### Describe the bug

Plugin skills installed via marketplace are visible in the `/skills` UI (showing all 18 skills correctly), but are **not** injected into the `` block in the agent's system prompt. Only the builtin `customize-cloud-agent` skill appears. When the agent calls the `skill` tool for any plugin skill, it returns `"Skill not found"`.

This differs from #2416 (sub-agent token truncation) — this affects the **main/default agent** and involves **zero** plugin skills appearing, not a truncated list.

### Environment

- **Copilot CLI version:** 1.0.28
- **OS:** Windows 11
- **Plugin:** `superpowers` v5.0.7 from `obra/superpowers-marketplace`
- **Install method:** Marketplace (`copilot plugin install` via `superpowers-marketplace`)

### Steps to reproduce

1. Install the superpowers plugin from marketplace
2. Verify `/skills` shows all 18 skills (17 plugin + 1 builtin) ✅
3. Start a conversation and ask the agent to invoke any plugin skill (e.g., `using-superpowers`)
4. Agent calls `skill` tool → returns `"Skill not found"`
5. Ask the agent what skills it can see — it only sees `customize-cloud-agent` (builtin)

### Evidence

The `~/.copilot/config.json` is correctly configured:

- Plugin is installed, enabled, `cache_path` points to the correct directory
- `disabledSkills` is empty
- Skills exist on disk at `{cache_path}/skills/` with valid `SKILL.md` files
- Plugin **agents** (e.g., `superpowers:code-reviewer`) ARE correctly discovered and available via the `task` tool

### Expected behavior

All enabled plugin skills should appear in `` and be invocable via the `skill` tool, matching what `/skills` shows in the UI.

### Actual behavior

Only builtin skills appear in ``. Plugin skills are invisible to the agent despite being correctly installed, enabled, and visible in `/skills`.

### Related issues

- #2416 — Sub-agent `` truncation (similar symptom, different scope)
- #2390 — Direct repo install path discovery issue (different root cause)

### Workaround

Manually read the `SKILL.md` files via the `view` tool and follow instructions, bypassing the `skill` tool entirely.