docs(skills): add the upstream EIP adoption skill

[pdobacz]

Capture the process for adopting one upstream EIP into a WIP Monad fork and releasing its fixtures.

Tested with 7976 and 2780 (because why not), seemed to work rather ok

Greptile Summary

The PR adds a reusable workflow for adopting an upstream EIP into a work-in-progress Monad fork and releasing its fixtures.

• Documents EIP scoping, branch setup, mixin-based fork construction, and explicit fork succession through follows().
• Covers spec porting, test selection and adaptation, fixture validation and release, and merge-time cleanup.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
.claude/commands/adopt-upstream-eip.md	Adds the end-to-end EIP adoption guide and replaces the unsupported class keyword with a classmethod-based succession mechanism, including instructions to add the supporting trait when the target branch lacks it.

_{Reviews (2): Last reviewed commit: "docs(skills): declare fork succession th..." | Re-trigger Greptile}