fix(tui): skip initial theme probe in tmux

[wgu9]

Issue for this PR

Closes #24475

Type of change

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

What does this PR do?

Avoids the blocking initial waitForThemeMode(1000) call when OpenCode starts inside tmux or screen-style multiplexers. Those environments can lose or delay OSC theme replies, which leaves startup waiting for the full timeout before the first draw.

The direct-terminal path still waits for the renderer theme mode once and falls back to dark if no response arrives. The multiplexer path uses dark immediately so the TUI can render without waiting on OSC replies.

How did you verify your code works?

• bun test test/util/theme-mode.test.ts from packages/tui
• bun typecheck from packages/tui
• bunx oxlint packages/tui/src/app.tsx packages/tui/src/util/theme-mode.ts packages/tui/test/util/theme-mode.test.ts from repo root (existing warnings only in app.tsx)
• git diff --check
• pre-push bun turbo typecheck

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Related PR Found:

• PR fix(tui): restore terminal capability detection over ssh #31947: fix(tui): restore terminal capability detection over ssh
  • URL: fix(tui): restore terminal capability detection over ssh #31947
  • Why related: This PR also deals with terminal capability detection and environment-specific behavior (SSH). While it's not a duplicate, it addresses similar terminal/environment interaction concerns in the TUI startup path.

The current PR (32523) is specifically focused on skipping the theme probe in tmux/screen multiplexers to avoid startup delays, while PR #31947 handles SSH-specific terminal capability detection. These are related but distinct fixes for different terminal environments.