refactor(execute): execution-unit cleanup + release-readiness fixes

[lukas-grigis]

Summary

Three commits on the back of the v0.6.2 setup/check-split branch:

• refactor(execute): slim per-task execution unit folder — drops per-unit tasks.json and prior-evaluations/; the canonical sprint-root copy + the inline ### Evaluator output block in tasks.md cover both.
• docs: replace stale evaluations/<taskId>.md path with per-round truth — CLAUDE.md / ARCHITECTURE.md / REQUIREMENTS.md now point at execution/<unit-slug>/rounds/<N>/evaluator/evaluation.md (the actual file EvaluateAndFixLoopUseCase writes) instead of the legacy sidecar path.
• fix(execute): release-readiness fixes for setup/check split — six audit findings against 0b6eece4..HEAD so v0.6.2 users can upgrade without losing sprints and the chain trace + adapter contracts match the tests that fence them.

Release-readiness fixes (last commit)

• Legacy sprint.json files load. Zod schema defaults setupRanAt to {}; v0.6.2 files carrying checkRanAt (the legacy key) parse successfully and self-clean on the next save. Two new sprint-schema tests cover the round-trip and the no-audit-key-at-all variant.
• Spawn-level setup failures hard-abort. setup-scripts-sprint-start now wraps the runSetupScript call in try/catch; ENOENT, EPERM, missing binary all surface as InvalidStateError({ currentState: 'setup-failed' }) naming the failing repo — identical to a non-zero exit. The soft OnError fallback (setup-scripts-sprint-start-noop) is gone — a broken baseline must fail loudly.
• Sprint resume skips already-stamped repos. The leaf no-ops per repo when Sprint.setupRanAt[repoPath] already carries a timestamp; the existing stamp is preserved.
• Identity-leaf helper consolidated. Single shared noopLeaf<TCtx>(name) at src/application/chains/leaves/noop-leaf.ts; the duplicate noopLeafExec / noopLeaf definitions in execute-flow.ts and per-task-flow.ts are removed.
• Tester agent memory line 232 references setupRanAt (post-rename field).
• Docs synced — CHANGELOG Unreleased, CLAUDE.md, ARCHITECTURE.md, REQUIREMENTS.md.

Pipeline (post-merge)

sprint start
  resolve-branch
  dirty-tree-preflight
  resolve-check-scripts
  setup-scripts-sprint-start    ← now hard-aborts on ANY setup failure
                                   (red exit OR spawn-level error);
                                   skips repos already stamped on
                                   sprint.setupRanAt (resume case)
loop per task:
  branch-preflight
  execute-task
  post-task-check
  evaluate-task
  mark-done

Test plan

• pnpm typecheck clean
• pnpm lint clean
• pnpm test — 233 files, 2262 tests pass (4 new: legacy-checkRanAt round-trip, no-audit-key fallback, hard-aborts on spawn error, skip-if-stamped + mixed-stamps)
• grep -rn "function noopLeaf" src/ returns exactly one match (the new shared module)
• grep -rn "setup-scripts-sprint-start-noop" src/ returns only the negative-assertion lines in the new test
• Manual smoke — RALPHCTL_ROOT=/tmp/legacy ralphctl sprint show against a hand-crafted v0.6.2 sprint.json with checkRanAt: {}: should load without error
• Manual smoke — kill sprint start mid-task, re-run: setup script does NOT re-execute on the stamped repo
• Manual smoke — point setupScript at a non-existent binary: chain hard-aborts naming the failing repo