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.

Start with the /skills listing and skill tool entry points, then compare how marketplace SKILL.md files under the configured cache_path are discovered with what populates the block. Confirm the fix by installing superpowers and verifying all 18 skills appear in the agent prompt and can be invoked through the skill tool.

Written by the indexing model from the issue text.