docs: add AGENTS.md for non-interactive / agent usage

[crowlbot]

Adds an AGENTS.md documenting how to drive the CLI non-interactively from an AI agent or CI job.

Covers:

• Token auth via DENO_DEPLOY_TOKEN (no browser), and whoami for a side-effect-free auth check
• --non-interactive / -y behavior and the env vars that back each prompted value
• --json structured output and the structured error envelope
• The exit-code taxonomy (0 OK / 1 GENERIC / 2 USAGE / 3 AUTH / 4 NOT_FOUND / 5 CONFLICT / 6 NETWORK)
• Global + per-subcommand flag reference
• 5 copy-pasteable example flows (auth check, create + deploy, prod deploy, env vars, list deployments)
• Idempotency / CONFLICT semantics

All flag names, env vars, exit codes, and JSON shapes were taken from the current source (config.ts, util.ts, auth.ts, deploy/*).