Summary

Relands BLOCKER-4 (delegated child-session empty-history fallback) reverted in #4044.
When a delegated child session fails on its first promptAsync call before history persists, runtime-fallback consumes the captured bootstrap retry parts (system + tools preserved) and clears the bootstrap entry to prevent double-consumption.

Why this reland sticks (vs PR #3825)

PR #3825's regression test was non-deterministic against post-#4032 schema changes and rotated logger state shared across tests. This version keeps the fix narrow:

• Only the empty-history branch consumes bootstrap.
• Clear-after-use invariant is asserted by an explicit test.
• logger.test.ts is rewritten to per-test isolated module imports, removing the rotation cross-talk that was one of the contributors to PR fix(delegate-task): harden child-session first-prompt fallback recovery #3825's failing root bun test (a major reason for Revert "Merge pull request #3825 from tw-yshuang/fix/delegated-child-session-early-failure-fallback" #4044).

Changes

• src/hooks/runtime-fallback/last-user-retry-parts.ts: when persisted history is empty and a bootstrap exists, return its retryParts and clearDelegatedChildSessionBootstrap(sessionID) to prevent reuse on subsequent retries.
• src/hooks/runtime-fallback/index.test.ts: add assertion that bootstrap is cleared after consumption.
• src/shared/logger.test.ts: per-test cache-busting dynamic import of ./logger so rotation state cannot leak across tests in the shared root suite.
• CHANGELOG.md: add v4.2.1 entry.
• docs/reference/known-issues.md: mark BLOCKER-4 RESOLVED in v4.2.1.

Testing

• bun test src/hooks/runtime-fallback/index.test.ts --timeout 30000 ✅ (63 pass / 0 fail)
• bun test src/shared/logger.test.ts --timeout 30000 ✅ (7 pass / 0 fail)
• Killer gate (the one that broke PR fix(delegate-task): harden child-session first-prompt fallback recovery #3825): bun test --timeout 30000 ✅ (7072 pass / 1 skip / 0 fail / 7073 across 729 files)
• bun run typecheck ✅ (clean)
• bun run build ✅ (clean)

Related Issues

• Resolves delegate-task child-session fallback can stop before empty-history retry #4059
• Supersedes (reverted) fix(delegate-task): harden child-session first-prompt fallback recovery #3825 and Revert "Merge pull request #3825 from tw-yshuang/fix/delegated-child-session-early-failure-fallback" #4044
• Plan: .omo/plans/p0-4059-blocker4-reland.md

Summary by cubic

Fixes BLOCKER-4 by making delegated child-session fallback work when the first prompt fails before history persists. On retry, we consume the captured bootstrap once, preserve system + tools, and clear the bootstrap immediately to prevent reuse.

• Bug Fixes
  • When history is empty, consume the delegated bootstrap retry payload once and clear it right away.
  • Added an explicit test asserting the bootstrap is undefined after consumption.
  • Isolated src/shared/logger.test.ts via per-test dynamic imports to stop rotation state leaks.
  • Updated CHANGELOG.md and marked BLOCKER-4 resolved in v4.2.1 docs.

^{Written for commit 971be27. Summary will update on new commits. Review in cubic}