add developer docs

docs-site/docs/developer/index.md

@@ -0,0 +1,25 @@
+# Developer hub (this repository)
+
+!!! warning "Not an open-source product repo"
+This tree is **reconstructed proprietary source**. You can maintain **documentation**, trace **read-only** flow in `src/`, and run small **helper scripts**. You **cannot** legally ship Claude Code from this mirror, and there is no supported way to compile the full CLI here.
+
+## What you can do
+
+| Activity                     | How                                                                                                                    |
+| ---------------------------- | ---------------------------------------------------------------------------------------------------------------------- |
+| **Preview or extend docs**   | [Editing documentation](editing-documentation.md)                                                                      |
+| **Understand control flow**  | [Navigating the source](navigating-the-source.md)                                                                      |
+| **Learn build-time gates**   | [Bun bundle and feature flags](bun-bundle-and-feature-flags.md)                                                        |
+| **Regenerate appendix data** | `scripts/gen-appendices.sh` at repo root (see [Appendix: environment variables](../appendix/environment-variables.md)) |
+
+## What you cannot do (from this mirror alone)
+
+- Run `npm install` / `bun build` for the full product (no root `package.json`, private graph, `bun:bundle` feature flags).
+- Execute a project-wide automated test suite for the CLI (no harness shipped in the reconstruction).
+
+Details: [Reproducibility and limits](../reproducibility.md).
+
+## Related
+
+- [System design: layers](../system-design/layers.md)
+- [Official docs map](../official-docs-map.md)