fix(opencode): pass EXA_API_KEY to websearch tool to avoid rate limits

[rasdani]

Issue for this PR

Closes #15953

Type of change

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

What does this PR do?

Adds the ability to pass EXA_API_KEY to websearch tool to increase rate limits. @thdxr

Uses https://mcp.exa.ai/mcp?exaApiKey=YOUR_EXA_KEY when the environment variable gets detected.

Documented in: https://exa.ai/docs/reference/exa-mcp#api-key

How did you verify your code works?

I tested it locally and didn't hit any rate limits anymore.

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

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

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Hey! Your PR title pass EXA_API_KEYtowebsearch tool 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. 👍