Buildkite adds pipeline-scoped secrets and agentic CI tools

Buildkite introduced secrets scoped to a single pipeline with conditions on branch, queue and build creator. Its open-source MCP server gained validate_pipeline, compare_builds and create_pipeline_webhook tools, and the GitHub Actions compatibility layer now supports Linux arm64.

• Pipeline secrets stay hidden after creation and need pipeline and cluster permissions
• validate_pipeline checks YAML against the schema without starting a build
• compare_builds diffs a failed build against the last successful one on the branch
• Agent v4.1.0 adds cache save and restore durations and sizes to telemetry