Improve goal completion usage reporting

[etraut-openai]

Why

Goal completion follow-up turns currently receive a preformatted English usage sentence such as time used: 2586 seconds. That nudges the model to echo an awkward raw seconds count in the final reply, even though the tool result already exposes structured usage fields like goal.timeUsedSeconds, goal.tokensUsed, and goal.tokenBudget.

What changed

• Replace the preformatted completion usage sentence with guidance to read the structured goal fields from the tool result.
• Preserve token-budget reporting while allowing the model to phrase elapsed time in a concise, human-friendly way that fits the response language.
• Update core coverage for both the generated completion guidance and the session flow that forwards it back to the model.

Verification

Previously, it would have output a final message indicating that it "worked for 303 seconds". Now it shows the following:

[fcoury-oai]

Approved.