[skills] Add offload-to-sandbox skill

[cramforce]

Adds a new skill, Offloading work to a Vercel Sandbox (skills/offload-to-sandbox). It's about using the vercel sandbox CLI as remote workers for heavy builds, full test suites and many parallel (agent) tasks, so they don't load the local machine. It complements vercel-sandbox, which covers running untrusted code through the SDK.

The content comes from a multi-week performance program in which a dozen coding agents shared one laptop and offloaded builds and sharded test suites to Vercel Sandboxes. It is written to be project-agnostic:

• when to offload and what to keep local
• a base snapshot built once (pinned toolchain, pre-built baseline tree, helper scripts)
• one sandbox per task: naming, sizing (2 GB per vCPU and the OOM risk), lifetime vs exec timeouts, tags
• pushing source as a tarball of git-known files while keeping node_modules and build artifacts
• running commands: quoting pitfalls (zsh globs and modifiers), script files, detached long jobs with an exit-status file
• getting results back as small files
• sharding suites; separating pre-existing failures from new ones against an untouched baseline
• auth, a small wrapper script plus a SANDBOX.md for agents, cost and cleanup, and designing for restarts

Also adds the required ⤳ skill: offload-to-sandbox node to the ecosystem graph in vercel.md.

bun run validate passes. The CLI flags were checked against vercel sandbox --help from Vercel CLI 62.1.0.