fix:Guard against missing agent, add warning & fallbacks, and fix dialog mouse selection handling

[yejiming]

Issue for this PR

Closes #19565
Closes #19591

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR fixes two TUI interaction bugs:

1. Prevents crashes when local.agent.current() is temporarily undefined (startup/race condition path, reported on Windows).

   • Guarded prompt submit paths and UI reads in prompt/index.tsx.
   • Added a warning toast and early return when no agent is available.
   • Updated agent dialog to safely pass current as optional.

2. Fixes dialog overlay dismiss behavior so clicking outside closes on the first click.

   • In ui/dialog.tsx, clearing selection no longer consumes the first outside click.
   • Non-left clicks are still ignored for dismiss.

These changes preserve existing behavior when data is ready, while avoiding undefined access and improving dialog UX.

How did you verify your code works?

• Ran bun typecheck in packages/opencode (fails in this environment because tsgo is not installed).
• Ran bun test test/cli/tui --filter dialog in packages/opencode (fails in this environment because @opentui/solid/preload is unavailable).
• Verified all changed call sites now handle missing agent defensively and compile logically.

Screenshots / recordings

N/A (TUI interaction fix; no browser UI screenshot).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Guard against missing agent, add warning & fallbacks, and fix dialog mouse selection handling doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[JosXa]

Please check if renderer.clearSelection() is no-op safe. It may not be necessary to check before clearing

[yejiming]

You’re right, renderer.clearSelection() is no-op safe. I’ve changed it to call renderer.clearSelection() directly.

[rekram1-node]

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