[Feature Request] Add "Copy At-Mention Reference" command in VSCode extension

Summary

Currently, Option+K (Mac) inserts an at-mention reference (e.g., @src/app.ts#5-10) directly into the Claude Code prompt. It would be useful to have a separate command/keybinding that copies the at-mention reference to the clipboard instead of inserting it into the prompt.

Use Case

• Reuse the same file reference across multiple prompts
• Share file references with teammates
• Paste the reference into other tools or documentation

Proposed Behavior

A new command (e.g., Copy At-Mention Reference) that copies the current file path and selected line range in at-mention format (@file.ts#L5-10) to the clipboard, without inserting it into the Claude Code prompt.