Chat input should insert file and code references inline

## Problem\nWhen files or folders are referenced in the chat input, they are currently represented primarily as attachment chips. This is less discoverable and less convenient than Cursor-style inline references. Partial code selections also need a readable inline representation.\n\n## Proposed behavior\n- Insert file references directly into the prompt, for example @src/file.ts.\n- Represent selected code ranges, for example @src/file.ts:2-4.\n- Represent folders as inline references, for example @src.\n- Make inline file/range references clickable so users can open the file and navigate to the selected range.\n- Preserve compatibility with existing drafts, history, and legacy attachment references.\n\nRelated PR: https://github.com/microsoft/vscode/pull/325949