Tool request: download workflow run artifact contents

提案されているエントリーポイント download_workflow_run_artifact(owner, repo, run_id, artifact_name) から始め、artifact の検出とダウンロードのために列挙されている REST エンドポイントを確認します。1 回の MCP ツール呼び出しで、選択した workflow artifact の内容（実行後の分析用の token-usage.jsonl を含む）を取得でき、呼び出し元が REST 呼び出しを連鎖させたり、artifact を自分で unzip したりする必要がなければ完了です。

Problem

For cost tracking and post-run analysis of agentic workflows, the missing piece is read access to workflow run artifacts. Specifically, downloading the agent-artifacts artifact from a completed gh aw run to parse token-usage.jsonl for per-run token cost data.

The current workaround requires chaining several REST API calls:

1. GET /repos/{owner}/{repo}/actions/runs/{run_id}/artifacts to list artifacts
2. GET /repos/{owner}/{repo}/actions/artifacts/{artifact_id}/zip to download
3. Unzip and parse the relevant file

This is a lot of plumbing for what should be a natural thing for an AI agent to do: "look at what the last run produced."

Proposed tool

download_workflow_run_artifact(owner, repo, run_id, artifact_name) that returns the artifact contents (or a structured representation for known formats like JSONL).

Why this matters

The token-usage.jsonl format written by gh-aw's firewall proxy (since v0.25.8) contains per-model token counts for every agentic workflow run. A single MCP tool call to retrieve it would unlock a natural class of "analyze what the last agent run did" workflows. Right now that analysis requires dropping out of MCP and into raw REST.

More broadly, artifacts are a primary output mechanism for GitHub Actions. Being able to read them via MCP would make a wide range of post-run analysis workflows significantly simpler.