Support `@include` / `extends` in instruction files to reference external URLs or repos

### Describe the feature or problem you'd like to solve

Instruction files are currently static and self-contained. There is no way to share common conventions across repos or teams without duplicating files — either manually copying them into every repo or requiring each contributor to set up `~/.copilot/instructions/` locally.

### Proposed solution

Introduce an `extends` or `@include` directive in instruction files (similar to Renovate's extends or ESLint's extends) that allows referencing external instruction files by URL or GitHub path:

```yaml
---
extends:
- "github:mu88/dotfiles/.copilot/instructions/dotnet.instructions.md"
- "https://raw.githubusercontent.com/org/shared/.github/instructions/security.instructions.md"
applyTo: "**/*.cs"
---
```

### Example prompts or workflows

1. A maintainer publishes shared .NET conventions in a dotfiles repo — every project references them with one line, zero duplication.
2. A new contributor clones a repo and immediately gets all team conventions — no local setup required.
3. An org updates shared security guidelines in one place; all repos pick up the change automatically.

### Additional context

`COPILOT_CUSTOM_INSTRUCTIONS_DIRS` already allows pointing to external directories, but requires manual per-machine setup. A declarative `extends` in the instruction file itself would be zero-config for contributors and composable by design.

Note: Organization-level instructions already exist for GitHub organizations. This request addresses the gap for personal account repositories and zero-config contributor onboarding.

Start by reviewing the existing instruction-file handling and the COPILOT_CUSTOM_INSTRUCTIONS_DIRS behavior mentioned in the issue. Define how the listed GitHub paths and URLs are resolved and verify that referenced instructions are applied alongside local files, including the YAML front matter examples shown here.

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