fix: white-on-white text in light terminal themes

[anandgupta42]

What does this PR do?

Fixes invisible (white-on-white) text when using light terminal themes with the experimental markdown renderer. Two root causes:

1. The <markdown> element was missing fg={theme.text}, falling back to OpenTUI's hardcoded white default (RGBA(1,1,1,1))
2. The markup.raw / markup.raw.block syntax scopes lacked a background property, so fenced code blocks had no contrast on light backgrounds

This is a well-known upstream issue (anomalyco/opencode#17323, #17935, #18353) with no merged fix.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Issue for this PR

Closes #617

How did you verify your code works?

• Added 30 E2E tests across 3 light themes (GitHub, Solarized, Flexoki) verifying:
  • markup.raw scope has background set
  • Default foreground is never white on light backgrounds
  • All syntax foregrounds have WCAG-derived contrast ratios
  • Dark mode regression checks
• bun run typecheck passes with 0 errors
• bun run build:local succeeds
• Full test suite: 6759 pass, 0 fail across 7240 tests

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• New and existing unit tests pass locally with my changes
• I have added tests that prove my fix is effective

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Multi-model code review: Reviewed by 6 models (Claude, GPT 5.2 Codex, Gemini 3.1 Pro, Kimi K2.5, MiniMax M2.5, GLM-5). Convergence: 1 round. fg type issue caught during convergence and fixed with @ts-expect-error.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 2e054cb1-bf77-4f4f-951e-2a81a43ce8e2

📥 Commits

Reviewing files that changed from the base of the PR and between 0d34855 and 67a5c34.

📒 Files selected for processing (3)

• packages/opencode/src/cli/cmd/tui/context/theme.tsx
• packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
• packages/opencode/test/cli/tui/theme-light-mode.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/light-theme-white-text

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