Add fast test target and speed up tests

[ryansimmen]

Summary

• Add make test-fast for a quick non-integration, non-race local test loop.
• Avoid unnecessary filesystem probes in runner tool tests.
• Parallelize isolated bounded-run and prompt integration cases to reduce race-suite wall time.
• Document the new fast test target in contributor and configuration docs.

Validation

• make verify
• make test-fast
• go test -race -count=1 ./internal/integration
• go test -race -count=1 ./...
• Stability runs for go test -race -count=1 ./...: 15.10s, 13.10s, 14.80s wall-clock