Startup banner 'N skill(s) failed to load' should name the failing skill(s)

## Problem

When Copilot CLI starts, it may display a banner like:

> ⚠️ 1 skill failed to load

The banner does **not** identify which skill failed. Running `/skills` also doesn't surface the error — every listed skill appears healthy, making it impossible to diagnose without digging into the process log files.

## Current behavior

- The `skills_loaded` telemetry event records `skill_error_count: 1` but no skill names.
- The only way to find the culprit is to grep the log for messages like `Skipping disabled MCP server: computer-use`.
- In this case the "failure" is a built-in MCP server (`computer-use`) that is intentionally disabled behind a feature flag, so the user can't act on it even after finding it.

## Expected behavior

1. **The startup banner should name the failing skill(s):**
`⚠️ 1 skill failed to load: computer-use (disabled)`

2. **`/skills` should have a way to surface errored/skipped skills** — e.g. a separate "Errors" section or a `/skills --errors` flag.

3. **Intentionally disabled built-in servers probably shouldn't count as errors at all** — a disabled-by-feature-flag server is not a user-actionable failure.

## Reproduction

1. Launch `copilot` (v1.0.45, enterprise plan, Windows).
2. Observe the "1 skill failed to load" banner.
3. Run `/skills` — no skill appears broken.
4. Grep the log: `Select-String -Path ~\.copilot\logs\process-*.log -Pattern "Skipping disabled"` → reveals `computer-use`.

## Environment

- Copilot CLI v1.0.45
- Windows 10 (10.0.26200)
- Node v24.15.0
- Enterprise plan

Aucun fichier d’implémentation ni test n’est nommé. Suivez la bannière de démarrage, l’événement de télémétrie skills_loaded, le rendu de /skills et la gestion des serveurs MCP désactivés ; le travail devra inclure des noms exploitables pour les skills en échec, un moyen de rendre visibles les skills ignorés ou en erreur, et aucun décompte d’erreurs pour les serveurs désactivés intentionnellement.

Rédigé par le modèle d'indexation à partir du texte de l'issue.