AI skill: validate Bicep template compatibility with azd local preflight snapshot

## Problem

Bicep template authors may unknowingly use patterns that prevent `bicep snapshot` from succeeding, which causes azd's local preflight checks (AI quota validation, reserved resource names, etc.) to be silently skipped. The most common problematic pattern is using non-null assertions (`!`) on module outputs:

```bicep
// ❌ This causes snapshot to fail — module outputs are null at snapshot time
principalId: myModule.outputs.systemAssignedMIPrincipalId!

// ✅ Use null-coalescing with a placeholder instead
principalId: myModule.outputs.?systemAssignedMIPrincipalId ?? '00000000-0000-0000-0000-000000000000'
```

## Proposed Solution

Create an AI skill (Copilot skill) that helps template authors validate whether their Bicep template can generate a snapshot for azd preflight checks. The skill should:

1. **Analyze the Bicep template** for patterns known to cause snapshot failures:
- Non-null assertions (`!`) on module outputs
- `reference()` calls to resources whose properties are only available after deployment
- Other patterns that produce `TemplateValidationException` during snapshot

2. **Recommend fixes** — suggest null-coalescing (`??`), safe navigation (`?`), or conditional expressions that allow the snapshot to succeed while preserving correct runtime behavior

3. **Run `bicep snapshot`** on the template and report whether it succeeds, providing actionable guidance if it fails

## Known Patterns That Cause Failures

| Pattern | Why it fails | Fix |
|---|---|---|
| `module.outputs.prop!` | Non-null assertion on unknown output | Use `module.outputs.?prop ?? placeholder` |
| `reference('res').prop` in nested module params | Runtime-only value | Use conditional or placeholder |

## Related

- Upstream: [Azure/bicep#18822](https://github.com/Azure/bicep/issues/18822)
- Investigation: #7986
- User warning: #8171
- Telemetry: #8172