【substack-mcp】サムネ付き記事投稿CLIを整備する

[nanameru]

Closes #1

概要

• Markdown本文からSubstack下書きを作成し、任意のカバー画像をアップロードして設定できる substack-publish-article CLIを追加
• --publish 指定時のみ公開し、--send-email / --share-automatically は --publish なしでは拒否する安全設計にした
• Fable記事本文 articles/fable5.md を追加し、実際にSubstack下書き作成とサムネ設定まで実行
• test-board.yaml とユニットテストを追加

実運用結果

• 下書きID: 204559799
• 編集URL: https://taiyoai.substack.com/publish/post/204559799
• カバー画像URL: https://substack-post-media.s3.amazonaws.com/public/images/f36d3f43-0553-48f5-a0a6-5b131cb50464_1456x819.jpeg
• 公開は未実行。プレビュー確認後に明示許可を得て --publish を付けて実行する。

検証

• .venv/bin/python -m unittest discover -s tests → 成功（2 tests）
• .venv/bin/python -m compileall src → 成功
• test-board once --board test-board.yaml → status=done:1

リスク

• Substackは公式公開APIではなく内部API依存のため、Substack側の仕様変更で失敗する可能性がある
• サムネイルPNGは既存 .gitignore の thumbnails/*.png 対象のためコミットしていない。下書きにはSubstack CDNアップロード済み画像を設定済み

[amazon-q-developer]

Review Summary

This PR adds a CLI tool for creating Substack articles with cover images, along with supporting infrastructure and tests. The implementation is well-structured with proper validation and safety mechanisms.

Critical Issue Found (1)

A security vulnerability in the exception handling that could leak sensitive information through error messages. The bare except Exception handler needs to be replaced with specific exception types to prevent exposing credentials or internal system details.

Code Quality Observations

The PR includes solid defensive programming practices such as blocked path prefixes for image uploads and proper validation of arguments. The test coverage demonstrates correct behavior for the main workflow and validation rules.

Action Required: Address the exception handling security issue before merging.

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[amazon-q-developer]

🛑 Security Vulnerability: Replace generic exception handler with specific exceptions to prevent leaking sensitive information. The current bare except Exception can expose credentials or internal system details if SubstackClient.from_env() or client methods fail with unexpected errors. Handle credential loading failures separately from API errors to provide safe error messages.

Suggested change

	except Exception as exc:
	print(f"error: {exc}", file=sys.stderr)
	return 1
	except RuntimeError as exc:
	# Credential loading or API initialization errors
	print(f"error: {exc}", file=sys.stderr)
	return 1
	except (ValueError, FileNotFoundError) as exc:
	# Validation or file errors
	print(f"error: {exc}", file=sys.stderr)
	return 1
	except Exception as exc:
	# Unexpected errors - don't expose details
	print(f"error: An unexpected error occurred during article processing", file=sys.stderr)
	return 1