Add dashboards for authors, tags, and recents

[HansF]

Summary

• derive author/tag aggregates and recent issue ordering from the fetched issues dataset
• add dashboard builders that write Markdown lists under Dashboards with incremental handling
• document the new dashboard outputs in export-obsidian.md

Testing

• not run (not requested)

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Codex Task

[chatgpt-codex-connector]

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