skills: add W&B reactive safety guidance

[ktaletsk]

Summary

• treat form submission and run-button clicks as the consent boundary for remote W&B writes
• document molab authentication and credential-safety expectations
• encode W&B remote-state ordering through explicit results or completion dependencies
• require synchronization before Artifact, Public API, or Registry consumers
• add idempotent re-submission and final-verification checks

Context

This PR is intentionally stacked on #647 and targets its wandbify_skills branch. It contains only skill guidance; no notebook conversions are included.

The additions capture recurring issues found while repairing converted W&B notebooks: marimo cannot infer ordering from mutations to run, config, Artifact, or remote W&B state, and reactive evaluation must not trigger remote writes before explicit user submission.

Validation

• quick_validate.py: skill is valid
• git diff --check: passed