v0.6.5: Renderer pipeline, docs rewrite, architecture diagrams

Lift block rendering to kernel SDK primitives (sw.renderers + sw.markdown)
so all surfaces share one markdown pipeline. Rewrite docs for external
audience — remove all fork history references. Add Mermaid architecture
diagrams, CONTRIBUTING guide, and extension tutorial.

- sw.renderers SDK module: kernel-level renderer registry
- sw.markdown SDK module: unified marked v16 + DOMPurify pipeline
- Browser extension script loader for renderer injection
- Notes + Docs surfaces migrated to sw.markdown.renderSync()
- 4 renderer extensions rewritten to IIFE + sw.renderers.register()
- 6 Mermaid diagrams in ARCHITECTURE.md
- CONTRIBUTING.md + TUTORIAL-FIRST-EXTENSION.md
- DESIGN-WORKFLOWS.md replaces fork-era design doc
- Surface alias routes removed from main.go
- ICD/SDK runners migrated to /admin/packages/ endpoints
- 13 new renderer tests
- Docs, CHANGELOG, ROADMAP cleaned of fork references

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CONTRIBUTING.md

@@ -0,0 +1,64 @@
+# Contributing to Switchboard Core
+
+## Development Setup
+
+**Requirements:** Go 1.23+, Node.js 20+ (for frontend tests), SQLite (local dev).
+
+**Build from source:**
+
+```sh
+cd server
+go build -o switchboard-core .
+DB_DRIVER=sqlite DATABASE_URL=/tmp/switchboard.db ./switchboard-core
+```
+
+**Docker (recommended):**
+
+```sh
+docker compose up --build
+# open http://localhost:3000  (default login: admin / admin)
+```
+
+Data persists in the `sb_data` named volume. Reset with `docker compose down -v`.
+
+## Project Structure
+
+```
+server/          Go backend (handlers, store, config, auth, workflow engine)
+src/js/          Browser SDK and built-in surface JS
+packages/        Extension packages (surfaces, libraries, browser extensions)
+  build.sh       Builds each subdirectory into a .pkg archive
+docs/            Documentation markdown (served at /api/v1/docs)
+k8s/             Kubernetes manifests and Helm chart
+```
+
+## Running Tests
+
+**Backend:** `cd server && go test ./...`
+
+**Frontend:** `node --test src/js/__tests__/`
+
+## Code Conventions
+
+- **Go:** Standard formatting via `gofmt`. Handlers in `server/handlers/`,
+  persistence in `server/store/`. Database access goes through the store
+  interface, never directly from handlers.
+- **Browser JS:** Vanilla ES modules + Preact via CDN. No build step for
+  browser code (except the CM6 editor bundle). SDK lives at `src/js/sw/`.
+- **Extensions:** IIFE pattern (see `packages/*/js/script.js`). Register with
+  `sw.renderers` or `sw.slots` via the `sw:ready` event.
+
+## Creating a Package
+
+A package is a ZIP archive (`.pkg`) containing `manifest.json` and optional
+`js/`, `css/`, `assets/`, and `script.star` files. The build script
+(`packages/build.sh`) automates this. See `docs/PACKAGE-FORMAT.md` for the
+full manifest spec and `docs/TUTORIAL-FIRST-EXTENSION.md` for a walkthrough.
+
+## Pull Request Process
+
+1. Create a feature branch from `main`.
+2. Make your changes. Keep commits focused.
+3. Run both Go and JS test suites and confirm they pass.
+4. Submit a PR with a clear description of what changed and why.
+5. Address review feedback, then squash-merge when approved.