feat(mdp): markdown preview with google-docs-style PR review

[yumafuu]

mdp renders GitHub-flavored markdown through the GitHub API and serves it locally. For a pull request it previews every changed markdown file and turns the page into a Google Docs-style review surface.

What it does

• Left sidebar — changed files (common path prefixes trimmed) with the active file's outline nested beneath it, scroll-spy included; click the file name to collapse.
• Right margin comments — review threads sit next to the text they reference. Hovering highlights the anchor, clicking scrolls to it.
• Line-less comments grouped at the top — the PR's conversation comments, plus any thread whose text can't be located, stack under 行に紐づかないコメント.
• Range comments — select text and comment on it. The body carries a quote plus a hidden <!-- mdp:range ... --> marker, so mdp re-highlights the exact range on the next launch while GitHub still shows an ordinary line comment. Selections outside the diff fall back to a PR comment.
• Reply / resolve / react — click a card to reply, the blue check resolves the thread, 🙂+ adds any of GitHub's eight emoji.
• Mermaid blocks rendered as diagrams, light/dark themes, font-size cycle, GitHub icon link back to the PR.

Verification

bun test — 152 passing; tsc --noEmit clean. Exercised end to end against real PRs: rendering, range posting (inline and PR-comment fallback), reply, resolve, reactions, and re-highlighting after restart.

Review note

Design docs for each iteration are under docs/superpowers/. This PR is also a convenient way to dogfood the tool: mdp <this PR url>.

🤖 Generated with Claude Code