Add Buildroot embedded Linux experiment

[xdotli]

Summary

• Successfully builds a bootable embedded Linux system using Buildroot for QEMU x86_64
• First-attempt success with zero errors and no human intervention
• Build time: 21 minutes 37 seconds (faster than expected)
• Generates 5.1MB kernel + 60MB root filesystem

Experiment Structure

embedded/build-buildroot/
├── README.md           # Overview, quick start, key learnings
├── EXPERIMENT.yaml     # Machine-readable metadata
├── artifacts/
│   ├── Dockerfile      # Ubuntu 22.04 with build dependencies
│   ├── build.sh        # Build orchestration with logging
│   ├── docker-compose.yml
│   ├── ARTIFACTS.md    # Documentation
│   └── output/         # Build outputs
│       ├── bzImage        # 5.1MB Linux kernel
│       ├── rootfs.ext2    # 60MB root filesystem
│       └── start-qemu.sh  # Auto-generated QEMU script
└── trajectories/
    └── SUMMARY.md      # Detailed agent trajectory

Key Metrics

Metric	Value
Agent	Claude Sonnet 4.5
Duration	~36 minutes
Outcome	SUCCESS
Build Time	21m 37s
Kernel Size	5.1 MB
Rootfs Size	60 MB

Key Learnings

1. Buildroot's defconfig system enables single-command configuration
2. Toolchain compilation (GCC) takes ~70% of build time
3. Total build faster than expected (22min vs 30-60min typical)
4. Well-designed build systems enable reliable automation

Test plan

• EXPERIMENT.yaml validates
• Artifacts are organized
• Trajectory is complete
• Build tested and produces bootable images
• QEMU boot test (optional verification)