`buildgraph test` — selective test suite detection

[bubunyo]

Same call graph analysis as analyze, but output which test packages/suites to run instead of which services to build. Selective testing is often a bigger CI time-saver than selective builds.

Acceptance criteria

• buildgraph test outputs affected test packages as JSON array
• buildgraph test --run directly invokes go test on affected packages
• Integrates with the same baseline workflow as analyze