Development environment setup

[masnwilliams]

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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Note

Low Risk
Low risk documentation-only change; no runtime behavior or API implementation is modified.

Overview
Adds two new "Key gotchas" to AGENTS.md: guidance for building the headful Chromium image (expected ~10 minute build, interactive vs detached run) and clarification of the /process/exec request/response schema (command as a string, args separate, stdout_b64/stderr_b64 base64-encoded).

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Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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