bootstrap: address Copilot review — wording/naming for timeout path

The one-shot retry path and the mid-batch subdivision now fire on receive-pack deadlines (408/504), not just size rejections, so the rejection-centric log message, the sizeIssue variable name, and the "rejected" wording in the actionable error were all misleading.

• Log message: "after target rejection" → "after batchable push failure" (the per-call reason field already distinguishes timeout vs rejection).
• Rename sizeIssue → subdivide, keyed on the shared remedy.
• Actionable error: "rejected the pack" → "could not receive the pack — too large, or too slow to receive within its deadline".