Add official SpacetimeDB agent setup guide

[cloutiertyler]

Description of Changes

Add docs/static/agent-setup.md as the official instructions for preparing an AI coding environment for SpacetimeDB. The guide covers CLI prerequisites, Claude Code and Codex plugins, Cursor and other-agent fallbacks, existing project instructions, MCP verification, and an honest complete/partial report. Link it from the README.

Website companion: https://github.com/clockworklabs/spacetime-web/pull/1364. It serves this file at https://spacetimedb.com/agent-setup.md from the existing public-repository cache. Merge this document before deploying that endpoint.

API and ABI breaking changes

None.

Rollback safety impact

n/a

Expected complexity level and risk

1 — documentation only. Setup preserves existing project files and separates installing guidance from logging in, starting servers, publishing, or operating a database.

Testing

• Checked CLI and plugin command syntax against installed SpacetimeDB, Claude Code, and Codex help, plus the official plugin manifests and READMEs.
• Ran skills discovery against the exact documented source (skills add ... --list): all 11 official skills found; none installed.
• Verified the installation link and used the repository MCP reference because the corresponding website route is not currently published.
• Markdown formatting and git diff --check pass.

[rekhoff]

Approved