refactor: use abstract ITEM placeholders with dynamic property mapping

Changed from fixed TASK placeholders to flexible ITEM placeholders that
automatically map all CSV fields to template placeholders.

Key changes:
- Renamed task-template → item-template in all templates
- Changed TASK_* → ITEM_* placeholder naming
- Implemented dynamic placeholder generation from item properties
- Any field in fieldMapping now creates ITEM_{FIELD} placeholder
- Updated all tests and documentation

Naming convention: CSV field → item.property → ITEM_PROPERTY
Example: "assignee" → item.assignee → {{ITEM_ASSIGNEE}}

This enables users to add custom fields without modifying generator code:
- Add "assignee": "Assignee" to fieldMapping
- Use {{ITEM_ASSIGNEE}} in template
- No code changes required

Benefits:
- More flexible and extensible
- Clearer abstraction (items vs tasks)
- Consistent naming convention
- Better documented

All 56 tests passing.

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

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

test/testHelpers.js

@@ -61,9 +61,9 @@ export const createSampleTemplate = () => `<svg xmlns="http://www.w3.org/2000/sv
     </g>
 
     <!-- Task template -->
-    <g id="task-template" style="display:none">
-      <circle cx="{{TASK_X}}" cy="{{TASK_Y}}" r="6" fill="#1976D2"/>
-      <text x="{{TEXT_X}}" y="{{TEXT_Y}}" font-family="Arial" font-size="11" fill="#424242">{{TASK_ID}} {{TASK_TITLE}}</text>
+    <g id="item-template" style="display:none">
+      <circle cx="{{ITEM_X}}" cy="{{ITEM_Y}}" r="6" fill="#1976D2"/>
+      <text x="{{TEXT_X}}" y="{{TEXT_Y}}" font-family="Arial" font-size="11" fill="#424242">{{ITEM_ID}} {{ITEM_TITLE}}</text>
     </g>
   </defs>
 
@@ -80,8 +80,8 @@ export const createMalformedTemplate = (missingElement = 'month-template') => {
     <g id="lane-template" style="display:none">
       <rect x="{{LANE_X}}" y="{{LANE_Y}}" width="{{LANE_WIDTH}}" height="{{LANE_HEIGHT}}" fill="#FAFAFA"/>
     </g>
-    <g id="task-template" style="display:none">
-      <circle cx="{{TASK_X}}" cy="{{TASK_Y}}" r="6" fill="#1976D2"/>
+    <g id="item-template" style="display:none">
+      <circle cx="{{ITEM_X}}" cy="{{ITEM_Y}}" r="6" fill="#1976D2"/>
     </g>
   </defs>
   {{MONTHS}}
@@ -92,14 +92,14 @@ export const createMalformedTemplate = (missingElement = 'month-template') => {
     <g id="month-template" style="display:none">
       <line x1="{{MONTH_X}}" y1="{{GRID_TOP}}" x2="{{MONTH_X}}" y2="{{GRID_BOTTOM}}" stroke="#E0E0E0"/>
     </g>
-    <g id="task-template" style="display:none">
-      <circle cx="{{TASK_X}}" cy="{{TASK_Y}}" r="6" fill="#1976D2"/>
+    <g id="item-template" style="display:none">
+      <circle cx="{{ITEM_X}}" cy="{{ITEM_Y}}" r="6" fill="#1976D2"/>
     </g>
   </defs>
   {{MONTHS}}
   {{LANES}}
 </svg>`,
-    'task-template': `<svg xmlns="http://www.w3.org/2000/svg">
+    'item-template': `<svg xmlns="http://www.w3.org/2000/svg">
   <defs>
     <g id="month-template" style="display:none">
       <line x1="{{MONTH_X}}" y1="{{GRID_TOP}}" x2="{{MONTH_X}}" y2="{{GRID_BOTTOM}}" stroke="#E0E0E0"/>