chore: migrate repository to standard knowledge base layout

skills/markdown-to-epub/references/index.md

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+# markdown-to-epub Reference Index
+
+This directory keeps long-form notes for the Markdown to EPUB skill. The entrypoint stays in `../SKILL.md`; operational behavior is implemented by `../scripts/build_epub.py`.
+
+## Navigation
+
+- `../SKILL.md`: triggers, boundaries, quick commands, and examples.
+- `../scripts/build_epub.py`: source of truth for CLI arguments, asset normalization, Calibre invocation, and EPUB inspection.
+- `../agents/openai.yaml`: optional agent metadata.
+
+## Script Contract
+
+- Source Markdown is read-only.
+- Build artifacts are written to `--build-dir`.
+- Local images are copied into the build workspace and normalized when the file signature proves a better extension.
+- Remote `http(s)` and `data:` image references are not downloaded.
+- Strict mode fails when local image references cannot be resolved.
+
+## Verification
+
+Run the builder, then inspect:
+
+```bash
+unzip -l ./book.epub | rg 'content.opf|toc.ncx|nav.xhtml|\\.(png|jpg|jpeg|webp|gif)$'
+```
+
+Also inspect `report.json` in the build directory for `missing_images`, copied assets, and conversion status.