Development environment setup (#168)

Update `AGENTS.md` with notes on headful image build time and
`/process/exec` API schema.

These additions clarify non-obvious gotchas encountered during
development environment setup, specifically regarding the extended build
time for the headful image and the expected schema for the
`/process/exec` API endpoint.

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk documentation-only change; no runtime code or behavior is
modified.
> 
> **Overview**
> Updates `AGENTS.md` with two additional development gotchas: how to
build the headful Chromium image (including expected long build time and
interactive vs detached run behavior), and a clarification of the
`/process/exec` request/response schema (`command` as a string, args in
`args`, and base64-encoded `stdout_b64`/`stderr_b64`).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Mason Williams <masnwilliams@users.noreply.github.com>

Author: masnwilliams

AGENTS.md

@@ -42,3 +42,7 @@ The Cloud VM runs inside a Firecracker microVM. Docker requires:
 8. **E2e tests** use `testcontainers-go` and require Docker + pre-built images. Set `E2E_CHROMIUM_HEADFUL_IMAGE` and `E2E_CHROMIUM_HEADLESS_IMAGE` env vars to point to the correct image tags.
 
 9. **Go version**: The project requires Go 1.25.0 (per `server/go.mod`). The system Go may be older — ensure `/usr/local/go/bin` is on PATH.
+
+10. **Build headful image**: `cd /workspace && DOCKER_BUILDKIT=1 docker build -f images/chromium-headful/Dockerfile -t kernel-headful-test .` — this takes significantly longer than headless (~10 min) due to Xorg dependencies, Mutter, and the WebRTC client build. The headful `run-docker.sh` runs interactively (`-it`); for background use, run with `-d` instead.
+
+11. **`/process/exec` API schema**: The `command` field is a single string (the binary name), not an array. Arguments go in the separate `args` array field. Response `stdout_b64` / `stderr_b64` are base64-encoded.