Support adding selected lines to Claude prompt

[tv42]

Apparently, Claude Code lets you refer to specific lines, not just files, with @filename#L100-110. I can't find official docs about this, but it's done in VSCode (via https://cuong.io/blog/2025/06/23-claude-code-ide-vs-code) and elsewhere (https://github.com/sortafreel/select-copy-line/blob/02d630b535e0be2c2f2ed69fe3990c9764c4e2b2/src/extension.ts).

Being able to tell Claude to "rewrite this to return error instead of panicking" would be useful. Kind of like Zed's inline assistant, but I don't see a way to hook that one up to Claude Code.