Refactor config schema to use output and add config migration command

[rokuosan]

Summary

This PR updates the configuration schema to better reflect the current responsibility of the tool.

• introduce output as the canonical config section
• rename config references from the legacy hugo-oriented structure to output.articles / output.images
• keep legacy hugo config readable for backward compatibility in v1.0.0
• add a migrate command to rewrite existing config files to the latest schema
• update README and docs to use the new config structure

Motivation

The existing hugo config name no longer matches the actual role of the application.

The tool is no longer tied to Hugo-specific usage. The config is really about output paths and naming rules, so output is a more accurate and extensible structure.

Backward Compatibility

• existing hugo config is still supported for reading
• newly generated and migrated config uses the output schema
• legacy hugo keys are now treated as deprecated and intended for future removal

Example

Before:

hugo:
  directory:
    articles: content/posts
    images: static/images/%Y-%m-%d_%H%M%S
  filename:
    articles: "%Y-%m-%d_%H%M%S.md"
    images: "[:id].png"
  url:
    images: /images/%Y-%m-%d_%H%M%S

After:

output:
  articles:
    directory: content/posts
    filename: "%Y-%m-%d_%H%M%S.md"
  images:
    directory: static/images/%Y-%m-%d_%H%M%S
    filename: "[:id].png"
    url: /images/%Y-%m-%d_%H%M%S

Validation

• updated CLI/config tests
• verified backward-compatible config normalization
• verified targeted core tests for output path resolution and article generation behavior

[chatgpt-codex-connector]

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Reviewed commit: b483588891

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[rokuosan]

@codex[agent] review

[Codex]

@codex[agent] review

I ran go test ./... on the branch (all green) and did a quick pass over the config normalization + migrate implementation/tests; nothing obvious to change right now. Latest commit on the branch: 7ef8a62.

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

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• Open a pull request for review
• Mark a draft as ready
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