Item detail: PR links, changed files, descriptive activity feed

JC request 2026-08-23. Per item, surface: (1) associated PR(s) — number, title, URL, merge state; (2) files changed (from the PR diff or commit range); (3) richer activity entries — current feed is bare event types (e.g. "agent:build-agent comment.added", "item.transitioned") with no substance. Each activity entry should carry a human-readable summary: comment.added → comment body preview; item.transitioned → from→to status + who/why; item.created → type/priority/title context. Needs: contract/openapi.yaml extension (PR association on Item or a links[] field + activity detail payload), capture path so agents attach PR + changed-files on self-report, and web item-detail rendering. Spans API + MCP tools + web.