Add markdown-driven roadmap page

[atlas-maxjb]

Summary

• replace the /roadmap redirect with a real public roadmap page in the website repo
• use the existing Sure marketing navbar, footer, logo, background, and Tailwind styling
• add the approved left status summary + right vertical phase spine layout with collapsed phases
• parse the canonical roadmap Markdown from the Sure repository at runtime
• add parser and controller coverage

Sister PR

The canonical content lives in the Sure repository: we-promise/sure#2931

This website PR intentionally does not duplicate docs/roadmap.md. By default it reads:
https://raw.githubusercontent.com/we-promise/sure/main/docs/roadmap.md

Override with SURE_ROADMAP_URL when needed for previews or testing.

GitHub Projects are intentionally not involved. GitHub issues/PRs can be attached later as supporting workflow links; roadmap changes are controlled by PR review of the Markdown file.

Ruby/Tailwind checks could not run in the current environment because those executables are unavailable. git diff --check passes.

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