fix: use front matter date for output path templates

[rokuosan]

Summary

• use front matter overrides before resolving article output datetime
• apply the resolved datetime to article paths, image directories, image filenames, and image URLs
• add regression coverage for page bundle paths and image filename templates

Root cause

The exporter resolved %Y/%m/%d/%H/%M/%S placeholders from the issue-created date before applying front matter overrides, so date in front matter did not affect output paths.

Validation

• go test ./pkg/core ./pkg/config ./cmd/cli/...

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 7ed287ed50

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".