fix: add GitHubMention component to handbook MDX components

[devin-ai-integration]

fix: add GitHubMention component to default MDX components

Summary

The handbook page at /company-handbook/how-we-work/work-styles was crashing with the error:

Expected component GitHubMention to be defined: you likely forgot

GitHubMention was defined in mdx-jobs.tsx and included in jobsMdxComponents, but the handbook pages use defaultMDXComponents from mdx/mdx-components.tsx — which didn't include it. This adds the missing import and registration.

Review & Testing Checklist for Human

• Visit /company-handbook/how-we-work/work-styles and confirm it renders without error and the GitHub mentions display correctly (avatars + linked usernames)
• Spot-check that other handbook pages still render fine (no regressions from the new import)

Notes

• Link to Devin Session
• Requested by @ComputelessComputer

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[devin-ai-integration]

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