fix(delegation): atomically serialize reopenParentFromDelegation

[edelauna]

Related GitHub Issue

Closes: #365

Description

Replaces the sentinel-based multi-step write in reopenParentFromDelegation (parent active → close child → child completed → clear sentinel) with a single atomicUpdatePair call that writes child=completed and parent=active in one lock acquisition — no intermediate state is ever persisted.

Key changes:

• TaskHistoryStore.atomicUpdatePair — new public method that updates two HistoryItems within a single withLock acquisition. Both updaters run before any I/O; both files are written before the lock releases. Extracted upsertCore private helper to share the write+cache logic without triggering onWrite/index-write per record.
• reopenParentFromDelegation steps 3–5 — removeClineFromStack (step 4) moved before the atomic write so any stale "active" status written by abortTask is immediately overwritten. Steps 3, 5, and 5B collapsed into one atomicUpdatePair(childTaskId, parentTaskId, ...) call. Falls back to updateTaskHistory for the parent if atomicUpdatePair fails.
• Tests — atomicUpdatePair unit tests added to TaskHistoryStore.spec.ts (happy path, concurrent upsert ordering, missing-ID throws, partial-failure/known-limitation, onWrite called once). history-resume-delegation.spec.ts updated to stub taskHistoryStore.atomicUpdatePair instead of updateTaskHistory; handoff-correctness and partial-failure tests added. nested-delegation-resume.spec.ts updated with taskHistoryStore stub.

Why child first: a crash after the child write but before the parent write leaves child=completed, parent=delegated — which reconcileDelegationState already handles (resumable). A crash after the parent write but before the child write would leave parent=active with child=active and no way to report back.

Test Procedure

cd src
npx vitest run __tests__/history-resume-delegation.spec.ts \
  __tests__/nested-delegation-resume.spec.ts \
  core/task-persistence/__tests__/TaskHistoryStore.spec.ts

All 6447 tests pass (npx vitest run).

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: No documentation updates required (internal persistence layer change).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Additional Notes

assertValidTransition referenced in the issue pseudocode is not implemented — the guard at the top of reopenParentFromDelegation (checking status === "delegated" || status === "active" and awaitingChildId === childTaskId) already enforces valid state before the updaters run, and no transition table exists in the codebase to validate against.

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[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/core/webview/ClineProvider.ts	85.71%	0 Missing and 1 partial ⚠️

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