Feature request: invoke multiple Agent Skills from any position in Copilot Chat messages

## Feature request: inline and multiple Agent Skill invocation

### Problem

Copilot Chat recognizes an Agent Skill slash command when `/` starts a message, but a slash reference typed after existing text is treated as ordinary text. This also prevents invoking multiple skills in one request.

### Reproduction

1. Open Copilot Chat or the Agents window.
2. Ensure an Agent Skill such as `code-review` and `test-runner` is available.
3. Type text first, then type `/` in the same message, for example:

```text
Review this change with /code-review and validate it with /test-runner.
```

4. Try to select both skills from autocomplete and submit the message.

### Actual behavior

Skill autocomplete/invocation is available when `/` starts the message, but inline slash references after existing text are treated as plain text. Multiple skills cannot be selected in one message.

### Expected behavior

- Typing `/` anywhere in the composer opens skill and prompt-file autocomplete.
- Multiple skills can be selected in one message.
- Selected skills are represented as structured references or chips.
- Existing leading slash-command behavior continues to work.

### Environment

- VS Code: `1.134.0` (`110a328ea54b42367b803ec53ee0bf52ef26b419`)
- OS: macOS arm64
- Surface: Copilot Chat
- Agent Skills enabled with `github.copilot.chat.skillTool.enabled`

No Dev Tools Console error is produced; the inline reference is simply handled as text.

Documentation:
- https://code.visualstudio.com/docs/agent-customization/agent-skills
- https://code.visualstudio.com/docs/chat/copilot-chat

This is a composer UX and skill-reference parsing request, not a model tool-calling or Agent Host API request.