feat(tui) display context window limit in token usage counter and sidebar

[Haltarys]

Issue for this PR

Closes #42929

Could also fulfill #13003 as well up to you.

Type of change

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

What does this PR do?

The TUI input bar showed context usage as 30.0K (15%), but the percentage had no visible reference for what it was relative to. This PR adds the model's context window limit to both places that counter is rendered, so the input bar reads 30.0K / 200K (15%) and the sidebar context panel reads 30,000 / 200,000 tokens.

The limit comes from the model's limit.context (models.dev or provider config). packages/tui/src/component/prompt/index.tsx hoists it into a contextLimit variable and renders tokens together with the limit and percentage when a limit is known. packages/tui/src/feature-plugins/sidebar/context.tsx does the same for the sidebar panel. Models without a known limit behave as before.

How did you verify your code works?

• bun run --cwd packages/tui typecheck (tsgo --noEmit) passes, as does the full turbo typecheck run by the pre-push hook.
• Built the binary with bun run script/build.ts --single; the built-in --version smoke test passes.
• Installed the built binary and used it in a real session — the input bar shows 30.0K / 200K (15%) and the sidebar shows 30,000 / 200,000 tokens.

Screenshots / recordings

Before:

30.0K (15%)

After:

30.0K / 200K (15%)

Can add a terminal screenshot if needed.

Checklist

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

[github-actions]

Hey! Your PR title [FEATURE] Display context window limit in token usage counter and sidebar 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 updating your PR! It now meets our contributing guidelines. 👍