docs: expand onboarding guide with setup steps and designer workflow

[devin-ai-integration]

Summary

Expands the previously sparse onboarding page in the company handbook (how-we-work/5.onboarding.mdx) with practical getting-started steps. The old page had empty GitHub/Cursor sections and only staging build info. Now includes:

• First Steps checklist (GitHub access, Infisical invite, read handbook, read dev docs, download staging build, set up dev env)
• Development Environment section with editor recommendation, quick-start repo setup for Mac/Linux, and Cursor troubleshooting tips
• Local Supabase and Auth section explaining that local Supabase (Docker) is used during dev, and how to use Infisical to inject dev-only GitHub OAuth credentials for login
• For Designers section covering what to explore first, the Figma-to-code workflow, key directories for UI work, and how to contribute to content/docs

Existing staging build download/trigger sections are preserved unchanged.

Updates since last revision

Based on a Slack thread where a new team member didn't realize dev docs existed and couldn't log in (expecting production Supabase keys):

• Added Infisical invite as step 2 in First Steps
• Added explicit "read the developer documentation" as step 4 in First Steps
• Added new Local Supabase and Auth subsection clarifying: no production keys needed, task supabase-start generates everything, and the infisical run --env=dev ... command enables GitHub OAuth locally
• Added a tip callout at the end of Troubleshooting pointing to the developer docs hub

Review & Testing Checklist for Human

• Verify the Infisical project ID is correct — The command infisical run --env=dev --projectId=87dad7b5-72a6-4791-9228-b3b86b169db1 --path="/supabase" -- task supabase-start was taken from a comment in the Taskfile. Confirm this project ID is correct and appropriate to publish in docs.
• Verify internal links resolve correctly — The page now links to /docs/developers/login in addition to /docs/developers/setup, /docs/developers/env, /docs/developers/run, /handbook/about/what-hyprnote-is, /handbook/beliefs/design-philosophy, /handbook/about/product-philosophy, and /handbook/how-we-work/cms. Check these on the preview deployment.
• Review "For Designers" section for accuracy — The design-to-code workflow (Figma → Slack → code → PR) was inferred from team context. Confirm this matches the actual intended workflow.
• Check that the setup commands are current — The Mac/Linux quick-start steps were pulled from the existing developer docs. Verify nothing is outdated.

Notes

Requested by @ComputelessComputer
Link to Devin run

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