docs: make snapshot releases require a clean tree and restore step

[arawrdn]

GN team,

PR Description

Summary

Updates the snapshot release instructions in AGENTS.md to make the workflow safer and less error-prone.

What changed

• Adds a required clean worktree check before running snapshot versioning.
• Clarifies that uncommitted changes should be committed or stashed first.
• Adds pnpm build to the snapshot publishing flow.
• Adds an explicit git restore . step after publishing to discard generated snapshot version changes.

Why

pnpm changeset version --snapshot mutates package versions and may update lockfiles. If the working tree already contains unrelated changes, cleanup becomes risky, and snapshot metadata can accidentally leak into real PRs or future releases.

Requiring a clean worktree makes the post-publish restore step safe, predictable, and easy for both humans and agents to follow.

Happy coding!