Support explicit inline custom agent invocation and agent chaining within prompts

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

Copilot CLI currently supports custom agents from .github/agents, but there is no clear, concise way to explicitly invoke a specific agent in the middle of a prompt while preserving the current conversation context and accumulated results. Today, users can select an agent before a task or rely on automatic delegation. However, complex workflows often require intentionally switching between specialized agents during a single task. For example, I may want to: Analyze a codebase using the default agent. Pass the findings to a custom architecture-review agent. Pass the review results to a migration-planning agent. Generate implementation tasks using another specialized agent. The lack of an explicit inline invocation mechanism makes these multi-agent workflows harder to express, less predictable, and more dependent on model inference.

### Proposed solution

Add inline agent invocation syntax that allows users to explicitly delegate part of a workflow to a named custom agent while preserving prior context and generated artifacts.

### Example prompts or workflows

// assuming that '$' would be a trigger to call existing agents selection, like '@' does for files

> Analyze the repository and identify technical debt.
>
> $agent: architecture-review
> Review the findings and propose architectural improvements.
>
> $agent: migration-planner
> Create a phased migration plan based on the review.

### Additional context

Custom agents are one of the most powerful extensibility features in Copilot CLI. Explicit agent invocation would make agent-based workflows easier to discover, document, reproduce, and share among team members.

It would also reduce ambiguity around whether Copilot has actually used a particular custom agent or simply attempted to perform the task itself. For advanced users who invest in creating specialized agents, deterministic invocation would significantly improve confidence and workflow predictability.

该 issue 将 .github/agents 和现有的 custom-agent 选择列为起点；检查 prompts 如何选择 agents，以及对话上下文和生成的 artifacts 如何得到保留。定义并验证用于调用命名 agents 以及串联其结果的内联语法，并通过示例 workflow 能够按预期稳定运行来展示完成情况。

由索引模型根据 Issue 内容生成。