Start-Copilot: expose copilot --usage-output-file (write usage JSON to a file)

[shmuelie]

Summary

Start-Copilot does not expose the Copilot CLI's --usage-output-file flag. It is only reachable via -RemainingArgs passthrough, so unlike the launcher's other scripting-oriented options (-Silent, -OutputFormat) it has no first-class parameter, help, or path handling.

Feature

copilot --usage-output-file <file> writes the final usage statistics as JSON to the specified file. It complements non-interactive/scripting runs (-Prompt, -Silent, -OutputFormat json) by capturing token/credit usage for later processing.

Observed on GitHub Copilot CLI 1.0.81.

Proposal / acceptance criteria

• Add a -UsageOutputFile <path> parameter that emits --usage-output-file <path>.
• Document that it is most useful with -Prompt (non-interactive mode).
• Comment-based help, README, and an [Unreleased] CHANGELOG.md entry.
• Tests asserting the flag + value are emitted.