[IDE] Add shortcuts + hints for Duo context menu navigation

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Problem to solve

It's not obvious how to go back or close the Duo context menu using only the keyboard.

See gitlab-org/editor-extensions/meta#223 for more details on the problem and user feedback.

Why it's important

Learning keyboard patterns improves workflow efficiency and will make Duo UX stickier.

Proposal

See: #546092 for additional details on backspace behavior in nested categories.

Scenario: Interact with the context menu via keyboard
  Given the user has the context menu open
  Then show keyboard shortcuts for available actions

Example

Current	Proposed

Important links

• Figma
• Prototype iteration 3 (to illustrate the concept, not a 1:1 spec)