feat: fix contents panel scrollbar and consolidate control architecture

## Major Changes
- Fixed contents panel scrollbar behavior to only span content area when reaching max-height
- Eliminated duplicate control files across testdrive-jsui/static/ and markitect/static/
- Consolidated all control files to single source of truth in capabilities/testdrive-jsui/js/controls/
- Refactored contents control to use proper base class architecture

## Technical Details
- Moved overflow-y: auto from control-content-container to control-content-body
- Updated all HTML templates and plugin references to use capabilities/ paths
- Enhanced resize handle positioning (moved from -4px to 1px/2px from right edge)
- Improved CSS flex layout with proper min-height: 0 constraints

## Files Affected
- 10 duplicate control files removed
- 8+ reference files updated with new paths
- CHANGELOG.md updated with all changes

This eliminates confusion about which files to edit and ensures the UI
behaves correctly when panels reach viewport height limits.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

capabilities/testdrive-jsui/js/controls/control-base.js

@@ -763,17 +763,22 @@ class ControlBase {
             content.innerHTML = `
                 <div class="control-content-container" style="
                     flex: 1;
-                    overflow-y: auto;
+                    display: flex;
+                    flex-direction: column;
                     margin: 0 0 10px 1rem;
                     padding: 0.75rem 1rem 1rem 0;
                     font-size: 0.8rem;
                     box-sizing: border-box;
                     min-height: 0;
                     border-radius: 0 0 6px 6px;
+                    overflow: hidden;
                 ">
                     <div class="control-content-body" style="
+                        flex: 1;
+                        overflow-y: auto;
                         padding: 0;
                         margin-bottom: 0;
+                        min-height: 0;
                     ">
                         ${innerContent}
                     </div>