Support bundled reference resources for plugin-installed custom agents in GitHub Copilot CLI

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

GitHub Copilot CLI plugins can include both custom agents and skills, but their resource-loading behavior appears inconsistent. Skills are documented as directory-based bundles. The official **Adding agent skills for GitHub Copilot CLI** documentation says that agent skills are “folders of instructions, scripts, and resources,” and that when a skill is invoked, Copilot “automatically discovers all of the files in the skill's directory and makes them available alongside the skill's instructions.”
Docs:
- https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/add-skills
- https://docs.github.com/en/copilot/reference/copilot-cli-reference/cli-plugin-reference
- https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/plugins-creating
Custom agents, however, are documented primarily as `.agent.md` profile files with YAML frontmatter and prompt content. Plugin docs show agents under an `agents/` directory, but do not describe a comparable resource-discovery model for files such as `agents/references/*.md`.
Docs:
- https://docs.github.com/en/copilot/how-tos/use-copilot-agents/cloud-agent/create-custom-agents
- https://docs.github.com/en/copilot/reference/custom-agents-configuration
- https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/plugins-creating
In practice, this means a plugin-installed skill can reliably reference bundled files with paths such as:
```text
references\template.md
scripts\helper.ps1
```
because those files live under the skill directory:
```text
skills/my-skill/
├── SKILL.md
├── references/template.md
└── scripts/helper.ps1
```
But a plugin-installed agent cannot reliably reference files under: `agents/references/template.md` using: `references\template.md` relative path from its `.agent.md` prompt. This creates friction for plugin authors who want to keep agent prompts concise and maintain reusable supporting materials, templates, checklists, examples, or routing guidance outside the main `.agent.md` file.

### Proposed solution

Add support for bundled resources for plugin-installed custom agents, matching the skill resource model as closely as possible.

For example, allow plugin agents to resolve relative paths from the containing agent directory:

```text
my-plugin/
├── plugin.json
└── agents/
├── my-agent.agent.md
└── references/
└── checklist.md
```

Then `my-agent.agent.md` could safely reference:

```text
references/checklist.md
```

Alternatively, document and support an explicit agent resource directory convention, such as:

```text
agents/my-agent/
├── AGENT.md
├── references/checklist.md
└── scripts/helper.ps1
```

If implementing bundled agent resources is not currently planned, please update the plugin and custom agent documentation to explicitly state that:

1. Plugin-installed agents are single-file runtime artifacts.
2. `agents/references/*` is not automatically discovered or made available.
3. Authors should inline required agent instructions or move reusable resources into skills.
4. Skills are the recommended mechanism for bundled scripts, templates, examples, and reference files.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by reading the plugin and custom-agent documentation linked in the issue, then inspect plugin.json and the agents/ layout to determine current resource resolution. Done means either bundled references resolve relative to the agent and the behavior is verified, or the limitation and recommended skills workflow are documented explicitly.

索引モデルが issue の本文から書いたものです。