feat: emit ::notice:: upgrade link when breaking changes are found

[reuvenharrison]

Summary

When the breaking action finds breaking changes, it now emits a ::notice:: annotation linking to a free review page at oasdiff.com/review. The page shows a side-by-side spec diff with the full change list, locked 🔒 approve/reject buttons, and an upgrade CTA.

Changes:

• breaking/Dockerfile: add jq (needed for URL encoding and PR head SHA extraction)
• breaking/entrypoint.sh: emit ::notice:: with free review URL only when breaking changes are found
• README.md: update oasdiff-token link to point directly to /pricing

Funnel flow

breaking changes found
  → ::notice::📋 Review & approve these breaking changes → oasdiff.com/review?...
    → free review page: side-by-side diff + change list + locked approve/reject
      → "Start free trial →" upgrade CTA

The notice only fires when there are actual breaking changes — clean runs stay silent.

Depends on: oasdiff/w3#21 (free review page), oasdiff/oasdiff-service#131 (POST /public/changelog)

Test plan

• Run against a spec pair with breaking changes — ::notice:: appears in Actions log
• Run against identical specs — no ::notice:: emitted
• Link in notice opens the free review page with the correct specs loaded

🤖 Generated with Claude Code