feat: end-of-run failures recap

[kevinccbsg]

Summary

• After the --- Run complete --- summary, when any tests failed, the CLI now prints a Failed tests (N): block listing every failed test (with its error) so the failure list survives tail -N truncation and is easy to copy as a single block.
• Green runs print nothing extra; abort path unchanged.
• 4 tests cover the behavior: recap on 2 failures, no recap on green run, test:fail without error field, multi-line error indentation.

Spec: docs/superpowers/specs/2026-05-08-failures-recap-design.md
Plan: docs/superpowers/plans/2026-05-08-failures-recap.md

Test plan

• Unit tests pass: npm test -- --run → 64/64
• Manual smoke: run a real suite with one intentionally broken test against a local relay; confirm recap appears at the very end and survives npx twd-relay run | tail -10
• Manual smoke: a fully green run prints no Failed tests block

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