docs: add Cursor Cloud environment setup instructions

[powoct]

What

Adds a ## Cursor Cloud specific instructions section to AGENTS.md capturing non-obvious setup/run caveats discovered while standing up the dev environment for cloud agents. No product code changed.

Why

Future cloud agents start from a snapshot with dependencies already installed. This documents durable, non-obvious gotchas so they can run/test the app without rediscovering them:

• Bun 1.3.14 is the runtime; non-login shells may not have it on PATH.
• Tests must run from package dirs, never repo root.
• bun run lint has a pre-existing baseline error/warnings on clean dev.
• No LLM provider keys are provisioned; how to wire a local mock OpenAI-compatible provider via OPENCODE_CONFIG_CONTENT to exercise the full agent flow.
• How to run the headless server (:4096) and the web UI dev server (:4444).

Verification

Set up the environment end-to-end in a cloud VM:

• bun install at root — OK
• bun typecheck — 30/30 packages pass
• bun run lint — runs (pre-existing baseline warnings + 1 error, unchanged)
• cd packages/opencode && bun test test/util — 119 pass
• Ran headless server + packages/app web UI in dev mode, configured a local mock model, and completed a hello-world chat exchange end-to-end.

Assistant reply in the web UI: Hello from the mock model! You said: "hello world". The opencode dev environment is working.

opencode_web_hello_world.mp4

hello world exchange

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