cross-references and mentions #6

#N in issue/MR text should link to the issue/MR, owner/name#N across repos, and @user should link to the owner page. Rendering-side first; backlinks and mention notifications can follow.

Shipped and live: autolink engine in internal/autolink (parser-based, never rewrites inside a/code/pre, only links targets that resolve) wired into issue/MR bodies, comments, and diff threads in c90a104. Viewer-aware: cross-repo refs to unreadable repos stay plain text. Backlinks and mention notifications remain future work.