chore(docs): add upstream adoption skill

[chaliy]

Summary

• Add an upstream adoption spec defining requirements for syncing changes from everruns/everruns.
• Add an adopt-upstream agent skill focused on goals: OpenAPI sync, delta analysis, obvious implementation, non-obvious proposals, and shipment.
• Register the new spec and skill in AGENTS.md.

Test Plan

• Tests pass locally (just test)
• Coverage ≥80% (not changed; docs/skill-only change)
• Linting passes (just pre-push)

Checklist

• Follows SDK API consistency guidelines
• Updated relevant specs (if applicable)
• Added/updated tests (not applicable; docs/skill-only change)
• Updated documentation (if applicable)