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>

docs/TUTORIAL-FIRST-EXTENSION.md

@@ -0,0 +1,119 @@
+# Build Your First Browser Extension
+
+This tutorial walks through building a browser extension that renders custom
+code blocks, modeled on the CSV Table Viewer that ships with Switchboard.
+
+**Prerequisites:** A running Switchboard instance, a text editor, and `zip`.
+
+## Step 1: Create the Directory
+
+```sh
+mkdir -p my-extension/js
+```
+
+## Step 2: Write the Manifest
+
+Create `my-extension/manifest.json`:
+
+```json
+{
+    "id": "my-extension",
+    "title": "My Extension",
+    "version": "0.1.0",
+    "type": "extension",
+    "tier": "browser",
+    "author": "you",
+    "description": "Renders demo code blocks as styled HTML",
+    "permissions": [],
+    "settings": {}
+}
+```
+
+- **id** -- unique identifier, used as the install key
+- **type** -- `extension` for browser-only packages; `full` or `surface` for
+  packages with backend routes
+- **tier** -- `browser` for client-side JS; `starlark` for server-side scripting
+
+## Step 3: Write the Browser Script
+
+Create `my-extension/js/script.js`. Browser extensions use the IIFE pattern
+and register with the SDK through `sw.renderers`:
+
+```js
+(function () {
+    'use strict';
+
+    function register() {
+        if (!window.sw?.renderers) return;
+
+        sw.renderers.register('demo-block', {
+            type: 'block',
+            priority: 10,
+            match(lang) {
+                return (lang || '').toLowerCase() === 'demo';
+            },
+            render(lang, code, container) {
+                container.innerHTML =
+                    '<div style="padding:12px;background:var(--bg-2);' +
+                    'border:1px solid var(--border);border-radius:8px">' +
+                    '<strong>Demo:</strong> ' + code +
+                    '</div>';
+            }
+        });
+    }
+
+    if (window.sw?._sdk) {
+        register();
+    } else {
+        document.addEventListener('sw:ready', register, { once: true });
+    }
+})();
+```
+
+The IIFE wrapper keeps variables out of global scope. `sw.renderers.register`
+takes a name and an options object: `type: 'block'` targets fenced code blocks,
+`match` checks the language tag, and `render` receives the language, raw code,
+and a container element. The `sw:ready` event fires once the SDK initializes;
+if already loaded, register immediately. Use CSS variables like `var(--bg-2)`
+and `var(--border)` to follow the active theme.
+
+## Step 4: Package It
+
+```sh
+cd my-extension
+zip -r ../my-extension.pkg manifest.json js/
+```
+
+The `.pkg` format is a ZIP with `manifest.json` at the root. Optional
+directories: `js/`, `css/`, `assets/`. If working inside `packages/`, use
+the build script instead: `bash build.sh my-extension`.
+
+## Step 5: Install It
+
+```sh
+curl -X POST http://localhost:3000/api/v1/admin/packages/install \
+  -H "Authorization: Bearer <token>" \
+  -F "file=@my-extension.pkg"
+```
+
+You can also install through the Admin UI under the Packages section.
+
+## Step 6: Enable and Test
+
+1. Open the admin panel, navigate to Packages, confirm "My Extension" is enabled.
+2. Go to any markdown surface (Chat, Notes, etc.).
+3. Enter a fenced code block with the `demo` language tag.
+
+You should see styled output instead of a plain code block.
+
+## Going Further
+
+- **Add CSS** -- create a `css/` directory; stylesheets are injected automatically.
+- **Post-renderers** -- register with `type: 'post'` to run after block renderers
+  finish (the Mermaid extension uses this for async SVG rendering).
+- **Settings** -- declare a `settings` object in the manifest for admin-configurable values.
+- **Server-side logic** -- set `tier: "starlark"` and add `script.star` for
+  backend API routes and database tables.
+
+See `PACKAGE-FORMAT.md` for the full manifest spec and `EXTENSION-GUIDE.md`
+for advanced patterns.