Test Run IDs tie parallel runs together

Set DONOBU_RUN_ID once in CI and every flow produced by that run — across parallel shards and workers — gets grouped under a single identifiable test run.

Flows now carry an optional run ID, sourced from the DONOBU_RUN_ID environment variable. Set it once in a CI step — a GitHub Actions run ID, a custom value, anything stable for that run and distinct from others — and every flow produced by that test run, including parallel shards and workers, gets grouped together under one identifier.

This shows up as a single row in the Test Runs view, rather than a scatter of individually timestamped flows you have to reconstruct into a run by hand. Runs that don't set the variable behave exactly as before.