Feature: file-reference keyboard shortcut (Cmd+Option+K / Ctrl+Alt+K) to insert @file references

## Summary

Request: add a keyboard shortcut in Junie (CLI / IDE integration) to quickly insert a
**file reference** into the prompt — e.g. `@path/to/file.ext#L1-99` — without typing
the path and line range by hand.

## Why

When pointing Junie at a specific file or line range, the path and `#Lstart-end` currently
have to be typed manually. A dedicated shortcut would:

- Speed up targeting the right code
- Reduce typos in paths and line ranges
- Bring Junie in line with the UX other AI coding integrations already offer

## Reference implementation

The **Claude Code [Beta]** JetBrains plugin ships exactly this and is a good model:

> **File reference shortcuts**: Use `Cmd+Option+K` (Mac) or `Ctrl+Alt+K` (Linux/Windows)
> to insert file references (e.g., `@File#L1-99`)

Plugin page: https://plugins.jetbrains.com/plugin/27310-claude-code-beta-

## Proposed

- `Cmd+Option+K` (macOS) / `Ctrl+Alt+K` (Linux & Windows)
- Inserts a file-reference token (`@#L-`) at the cursor in the active prompt
- Defaults to the currently active/selected editor file, using the current selection's
line range when text is selected

Happy to help refine the token format to match Junie's existing `@`-mention conventions
if they differ.

Start by locating Junie's active prompt entry point in the CLI/IDE integration and reviewing its existing @-mention conventions. Compare the behavior with the linked Claude Code Beta reference, including the platform-specific shortcuts and selected line range. Done means the shortcut inserts the agreed file-reference token for the active or selected file in both supported environments.

Written by the indexing model from the issue text.