[IDE] Update Duo placeholder text
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# Problem to solve
Without hints, users will have trouble learning how to toggle chat and trigger the context menu using the keyboard.
See https://gitlab.com/gitlab-org/editor-extensions/meta/-/issues/223 for more details on the problem and user feedback.
## Why it's important
Users that learn keyboard shortcuts for repeated Duo workflows will be more efficient thereby increasing Duo adoption/stickiness.
## Proposal
Blocked by https://gitlab.com/gitlab-org/gitlab/-/issues/544992
| Current | Proposed |
| ------ | ------ |
| {width="300"} | {width="300"} |
## 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 iteration 3](https://prototyping-939024.gitlab.io/) (to illustrate the concept, not a 1:1 spec)
issue