/clear vs /new unclear

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

Clarify /clear vs /new tooltips

### Proposed solution

The help tooltips for /clear and /new don't make it obvious that /new preserves the session for /resume, while /clear discards it permanently. Consider clarifying this distinction.

### Example prompts or workflows

_No response_

### Additional context

_No response_

The issue points to the help tooltips for /clear and /new; start by locating those command definitions or help text in the copilot-cli repository. Update the wording so /new's resumable session and /clear's permanent discard are explicit, then verify both tooltips and the /resume workflow.

由索引模型根据 Issue 内容生成。