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>

packages/chat/js/main.js

@@ -258,7 +258,8 @@
                 <${Button} size="sm" onClick=${saveEdit}>Save<//>
               </div>
             </div>
-          ` : html`
+          ` : msg.content_type === 'markdown' && sw?.markdown?.ready ? html`
+            <div class="chat-msg__content" dangerouslySetInnerHTML=${{ __html: sw.markdown.renderSync(msg.content, { sanitize: true }) }} />` : html`
             <div class="chat-msg__content">${msg.content}</div>`}
           <div class="chat-msg__meta">
             <span class="chat-msg__time">${timeAgo(msg.created_at)}</span>