Complete library cleanup and documentation overhaul

## Library Code Cleanup (~1,200+ lines removed)
- Remove legacy markdown system (markdown.js, previewer.js)
- Delete unused EditContext code from ui/editor.js (~400 lines)
- Remove version history UI components from form-renderer.js (~180 lines)
- Clean unused CSS styles from insertr.css (~120 lines)
- Update package.json dependencies (remove marked, turndown)

## Documentation Updates
- README.md: Update from markdown to HTML-first approach
- AGENTS.md: Add current architecture guidance and HTML-first principles
- TODO.md: Complete rewrite with realistic roadmap and current status
- demos/README.md: Update for development demo server usage

## System Reality Alignment
- All documentation now reflects current working system
- Removed aspirational features in favor of actual capabilities
- Clear separation between development and production workflows
- Accurate description of style-aware editor with HTML preservation

## Code Cleanup Benefits
- Simplified codebase focused on HTML-first approach
- Removed markdown conversion complexity
- Cleaner build process without unused dependencies
- Better alignment between frontend capabilities and documentation

Ready for Phase 3a server updates with clean foundation.

lib/src/core/editor.js

@@ -155,8 +155,8 @@ export class InsertrEditor {
             return 'link';
         }
         
-        // ALL text elements use markdown for consistent editing experience
-        return 'markdown';
+        // ALL text elements use text content type
+        return 'text';
     }
     
     handleCancel(meta) {

lib/src/core/insertr.js

@@ -135,7 +135,7 @@ export class InsertrCore {
         const tag = element.tagName.toLowerCase();
         
         if (element.classList.contains('insertr-group')) {
-            return 'markdown';
+            return 'group';
         }
         
         switch (tag) {
@@ -146,9 +146,9 @@ export class InsertrCore {
             case 'a': case 'button':
                 return 'link';
             case 'div': case 'section':
-                return 'markdown';
+                return 'text';
             case 'span':
-                return 'markdown'; // Match backend: spans support inline markdown
+                return 'text';
             default:
                 return 'text';
         }