feat(marketing): add Linear integration page

- Create /linear marketing page mirroring /slack patterns
- Add LinearIssueDemo animated component showing issue-to-PR workflow
- Add "Roo Code for Linear" to Product dropdown in nav-bar (desktop + mobile)
- Use @roo Code terminology (not @roomote)
- Include SEO metadata, OpenGraph, and Twitter card support
- Demo section has id="demo" with video placeholder for when YouTube ID is available
- CTAs link to CLOUD_APP_SIGNUP_HOME (hero) and CLOUD_APP_TEAM_TRIAL (final CTA)

Closes MKT-4

Author: roomote