fix(tui): keep command palette available in questions

[opencode-agent]

Summary

• make the command palette shortcut mode-independent so it works while question prompts are active
• keep the shortcut disabled while a modal dialog is already open
• extend the TUI keymap test coverage for command palette bindings outside base mode

Tests

• bun test test/cli/tui/keymap.test.tsx
• bun typecheck
• pre-push hook: bun turbo typecheck

[opencode-agent]

TUI screenshot proof

agent-tui raw ANSI stream rendered through xterm.js and captured with Playwright:

Asset is attached to the PR-scoped prerelease: https://github.com/anomalyco/opencode/releases/tag/pr-29948-screenshots