## 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

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] 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
e