fix: apply the review items for #685 at merge

palette agent switch: aria-hidden on .command-palette-agent, arrows lose role="button" and aria-label (no nested buttons in the row)
i18n: zh-CN entry for the "Agent for the new session (Left/Right to change)" tooltip
docs: Keyboard-Shortcuts Ctrl+K row says Left/Right change the agent on the New session row
test: pin the aria-hidden switch and the zh-CN tooltip entry in command-palette-ui.test.ts

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/wiki/Keyboard-Shortcuts.md

@@ -8,7 +8,7 @@ Press `Ctrl+?` in the app for the same list in a floating overlay.
 
 | Shortcut                        | Action                                                          |
 | ------------------------------- | --------------------------------------------------------------- |
-| `Ctrl+K` (also `Cmd+K`, `Alt+K`)| Find an open session or start a new one.                         |
+| `Ctrl+K` (also `Cmd+K`, `Alt+K`)| Find an open session or start a new one. On the **New session** row, `Left` / `Right` change the agent it starts. |
 | `Ctrl+Tab`                      | Next session.                                                    |
 | `Alt+[` / `Alt+]`               | Previous / next tab.                                             |
 | `Alt+1` to `Alt+9`              | Switch to tab N. Physical keys, so macOS Option layouts work.    |