fix: clarify contributing-run metrics in GitHub Release copy

[AllenXiao95]

Follow-up to #39 / PR #42.

Problem

The manifest already labels aggregate metrics with translation_metrics_scope=contributing_run_execution, but the human-readable GitHub Release body still renders generic lines such as 翻译条数 and 消耗 token. That can be read as exact final-artifact distinct-unit or lifetime-cost attribution, which #39 intentionally does not provide.

Fix

When translation_metrics_scope == contributing_run_execution:

• add an explicit statement that metrics are aggregated over contributing Provider runs and are not exact final-artifact distinct-unit/lifetime-cost attribution;
• show the number of contributing Provider runs when available;
• rename the ambiguous 翻译条数 line to Provider 词条执行范围（累计）;
• keep the existing API 调用, 涉及翻译资源, and token values, now under the explicit execution-evidence scope.

For manifests without the new scope, preserve the historical copy rather than silently reinterpreting old metrics.

Scope

Presentation only. No change to translation-evidence.json, aggregation, manifests, Provider accounting, per-unit attribution, artifact diffing, or publisher behavior.