Context

Fixes agent panel keybindings on Linux, mirroring the Windows fix from #43692.

On Linux, Ctrl+Y (the previous agent::AllowOnce binding) is intercepted by the focused text input as "redo", so the shortcut did nothing when the message editor was focused. This is the same issue fixed for Windows in #43692.

Changes (Linux and Windows):

• agent::AllowAlways: added shift-alt-q (was unbound on all platforms)
• agent::AllowOnce: ctrl-y → shift-alt-a (Linux); already shift-alt-a on Windows
• agent::RejectOnce: shift-alt-x on both platforms — shift-alt-z conflicted with agent::RejectAll bound in other contexts, causing the keybinding hint to not appear in the UI
• agent::ToggleNavigationMenu: ctrl-shift-j → shift-alt-j (Linux)
• agent::ToggleOptionsMenu: ctrl-alt-i → shift-alt-i (Linux)

Closes #52472

How to Review

Two files changed: assets/keymaps/default-linux.json and assets/keymaps/default-windows.json. Check the AgentPanel context block in each.

Self-Review Checklist

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Note : Reopens previous work from closed PR #52479 (fork was deleted)

Release Notes:

• Fixed agent panel Allow, Always Allow, and Reject keybindings not working when the message editor is focused on Linux
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