fix(task-history): atomic per-task merge and drop shared index file (#1231) (#1261)

* fix(task-history): prevent concurrent index clobbering

* test(task-history): add regression coverage for self-healing index recovery

* fix(task-history): atomic read-modify-write
  prevents cross-process lost updates

* fix(task-history): atomic read-modify-write
  prevents cross-process lost updates

* fix(task-history): drop _index.json, scan task dirs on read

* refactor(task-history): extract computeDelta and mergeWithDisk helpers

* fix(safeWriteJson): propagate non-ENOENT read errors during merge

* fix(task-history): harden cross-process merge and reconciliation

* fix(task-history): throw DeltaRejectedError on invalid merge and harden edge cases

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Co-authored-by: Naved  Merchant <naved.merchant@gmail.com>