perf: window review timeline context hydration

[stainlu]

Summary

• Bound issue/PR timeline hydration with GitHub Link header pagination instead of fetching every timeline page before prompt compaction.
• Preserve ClawSweeper's existing head/tail context shape: earliest events plus latest timeline activity, with an omitted marker in the rendered prompt context.
• Surface timeline hydration telemetry in review artifacts, matching the existing comments/files/commits count style.
• Include dashboard/ in the CI sparse checkout so test/dashboard-worker.test.ts can import the dashboard worker during pnpm check.

Why

collectItemContext currently full-paginates issues/{number}/timeline, then keeps only 80 compacted entries. On large OpenClaw threads that means N GitHub REST calls even though the model receives a small prompt window.

With this change, large timeline hydration uses the first page's Link header to jump to the final page range. A 3,000-event timeline goes from 30 timeline API calls to 2 calls for the usual 80-entry prompt window, while keeping the first 40 and latest 40 entries.

Verification

• pnpm run build
• pnpm run test:unit
• pnpm run format:check
• pnpm exec oxfmt --check --threads=1 .github/workflows/ci.yml src/clawsweeper.ts test/clawsweeper.test.ts
• git diff --check
• pnpm run check progressed through active-surface, limits, build:all, lint, and unit tests locally, then failed under local Node 22 in existing repair notifier tests that require the repo's Node >=24 runtime.