Add reusable Maya plugin e2e testing skill

[BramVR]

What to build

Add a reusable Codex skill for real Maya plugin UI/e2e proof. The skill should capture the recurring Maya Stall workflow for plugin artifacts, Windows Maya hosts, trusted plugin staging, screenshots/recordings, Scenario Result JSON, popup diagnosis, and exact-head PR closeout.

This is split out from rigging/gg_klv_push issue 17 so the reusable workflow lives in its own repository issue/thread/PR instead of being mixed into a product issue worker.

Acceptance criteria

• A maya-plugin-e2e skill exists under skills/ with valid frontmatter.
• The skill covers plugin artifact hash/size checks, host/run preflight, trusted plugin staging, UI proof checkpoints, screenshot/recording requirements, popup handling, failure classification, and closeout proof.
• The skill explicitly treats Maya Untrusted Plugin Loading as a trusted-staging/config failure, not accepted proof.
• Existing local unrelated edits in mac-agent-scripts are preserved and not included in the PR.

Required proof

• Fake/local gate: YAML/frontmatter parses and the skill can be read by Codex.
• Real gate: Not required; this is documentation/workflow capture for repeatable real Maya plugin proof.
• Merge rule: do not merge if unrelated dirty files are included or the frontmatter is invalid.

Blocked by

None - can start immediately.