Context

Raised by @martin-rueegg in PR #1261 (comment).

safeWriteJson currently returns Promise<void>. When a merge callback is provided, the caller has no way to know what was actually written to disk without re-reading the file or capturing the merge result via a closure (the current workaround in writeTaskFile).

Proposal

Change the return type to Promise<SafeWriteResult>:

interface SafeWriteJsonResult {
  bytesWritten: number
  dataWritten: unknown
}

dataWritten is the value passed to JSON.stringify after the merge callback runs. Callers can use it to update their in-memory cache without a closure side-effect or an extra read.

Why

• Eliminates the closure-capture pattern in TaskHistoryStore.writeTaskFile (lines 885-893)
• Enables future consumers of safeWriteJson to synchronize their cache without a workaround
• Aligns with the broader goal of making the merge result a first-class return value

Scope

• Update safeWriteJson signature and implementation
• Update all call sites (TaskHistoryStore, migration, import)
• Update tests

Related: #1231, PR #1261