[buildkite/skills]: putting `agents:` inside `matrix.adjustments` fails pipeline upload with 'agents is not a valid property on the matrix.adjustments configuration'. Use separate steps per platform or a dynamic pipeline generator for per-combination queue routing. Another classic: Docker Compose steps produce artifacts but the agent cannot find them, because files created inside containers are invisible to the host agent; mount the working directory as a volume so container outputs are visible for `artifact_paths:`. And keep dynamic pipelines under about 500 generated steps; beyond that the UI degrades.

Context: Two pipeline-config mistakes that produce confusing errors, from Buildkite's own skill guidance.

## Matched source
Source: Source: https://github.com/buildkite/skills/blob/HEAD/steering/pipelines.md
Original query: "Buildkite: agents property inside matrix.adjustments breaks pipeline upload"
Key terms: adjustments, agents, breaks, buildkite, inside, matrix, pipeline, property, upload