[IDE] Trigger context menu via button or typing +
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# Problem to solve
Reduce the friction around adding a context source by switching from typing `/include` to typing `+` or clicking on a button.
See https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/issues/1840 for more details on the problem and user feedback.
## Why it's important
Users will frequently add context sources when using Duo in the IDE.
## Proposal
1. Add a button (`+ Context`) that triggers the menu
2. Trigger the menu if the user types `+` (or other symbol like `@`, see https://gitlab.com/gitlab-org/gitlab/-/issues/555860)
## Example
[<img src="/uploads/0328089ea5368ffebc68eac68920660b/Screenshot_2025-05-21_at_7.55.29_PM.png" width="300" alt="Screenshot_2025-05-21_at_7.55.29_PM">](https://gitlab.com/gitlab-org/gitlab/-/issues/544992/designs/ContextMenu.png)
## Important links
- [Figma](https://www.figma.com/design/py1XaoxXhPpc1jFocBMUnx/%E2%AD%90-Duo-in-the-IDE--16983?node-id=3581-8990&t=qlJE8xkWORZTMdcK-1)
- [Prototype](https://prototyping-939024.gitlab.io/) (to illustrate the concept, not a 1:1 spec)
issue