Fit priority label descriptions to GitHub limit

[Takhoffman]

Summary

• Shorten the ClawSweeper P0-P3 GitHub label descriptions so each is at or below GitHub's 100-character limit.
• Add a focused unit test that enforces the GitHub label description limit for the priority label scheme.
• Keeps triagePriority-driven P0-P3 label syncing behavior unchanged.

Root cause

GitHub label descriptions are limited to 100 characters. The newly added priority label descriptions exceeded that limit, so apply-decisions failed while creating labels, as seen in the ClawSweeper run for openclaw/openclaw#82625.

Validation

• pnpm run format
• pnpm run build
• node --test test/clawsweeper.test.ts
• git diff --check
• pnpm run check