fix(opencode): incorrect light mode detection in terminals that misreport CSI mode 997

[GuillenSanti]

Issue for this PR

Closes #20926

Type of change

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

What does this PR do?

Ghostty (and potentially other terminals) responds to CSI mode 997 with "light" even when the actual terminal background is dark (#000000). Since @opentui/core gives CSI priority over OSC, the correct background color inference is discarded and OpenCode renders in light mode.

I traced the issue by logging the renderer internals after waitForThemeMode resolves:

source=csi, oscBg=#000000, oscFg=#f8f8f2

CSI says "light" but the real background is black — a clear contradiction. This happens even with window-theme = dark explicitly set in the Ghostty config, so it's not a user misconfiguration.

The fix adds a resolveThemeMode() function that cross-checks the CSI result against the actual OSC 10/11 background luminance (using the same weighted brightness formula as @opentui/core). When they disagree, the objective color measurement wins.

The function is a no-op when:

• Detection source wasn't CSI (no override needed)
• No OSC background color available (can't cross-check)
• CSI and OSC agree (no contradiction)

How did you verify your code works?

1. Ran bun run dev in Ghostty with background=#000000 → correctly renders in dark mode (was light before)
2. Changed Ghostty config to background=#ffffff → correctly renders in light mode (not forced dark)
3. bun typecheck passes across all packages

Screenshots / recordings

N/A — terminal-only change, behavior verified interactively.

Checklist

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

[github-actions]

Hey! Your PR title Fix incorrect light mode detection in terminals that misreport CSI mode 997 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.

[github-actions]

Thanks for your contribution!

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

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