bench/precision-report.ts prints the weekly precision comment from /precision history #452

Part of #439 (one comment on #439 each week: the scorecard trend, the worst joint, the next experiment).

Done:

• [ ] bun bench/precision-report.ts <url> reads the hourly roll-up that #436 keeps. It prints a markdown comment with each headline number this week against last week, the worst joint, and the open experiment (from a line the crew edits in Software/README.md).
• [ ] A test feeds it a fixed history and checks the output.
• [ ] It only reads; it posts nothing itself.