Add GitHub link to header help (?) menu

[blimmer]

Summary

The plan UI's header help menu (the ? CircleHelp icon, top-right) currently exposes two items: Documentation and Get Help in Slack. Add a third item that links to the project's GitHub repository so users have an obvious path to file issues, browse source, and star the project.

Where

• Component: packages/ui/src/components/Header.tsx
• Test ids exported from the same file (extend headerTestIds with a new helpGithubItem entry)
• Shared link constants: packages/shared/src/links.ts — add a GITHUB_REPO_URL (or similar) pointing to https://github.com/contextbridge/planbridge
• Consumer: packages/plan/src/App.tsx passes the new prop to <Header /> (next to docsHref and slackHelpHref)

Suggested item

• Label: View on GitHub (or GitHub)
• href: the new shared GITHUB_REPO_URL
• target=\"_blank\" + rel=\"noopener noreferrer\" to match the existing items
• Place after Documentation and before Get Help in Slack (or at the end — design call)

Acceptance criteria

• A new dropdown item appears in the help menu pointing at the GitHub repo
• Header tests in packages/ui (and any plan-level integration tests that snapshot the header) cover the new item via a new headerTestIds entry
• just verify passes