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>

generator.js

@@ -54,13 +54,13 @@ window.timelineGenerator = {
   validateTemplate(template) {
     const hasMonthTemplate = template.includes('id="month-template"');
     const hasLaneTemplate = template.includes('id="lane-template"');
-    const hasTaskTemplate = template.includes('id="task-template"');
+    const hasItemTemplate = template.includes('id="item-template"');
 
-    if (!hasMonthTemplate || !hasLaneTemplate || !hasTaskTemplate) {
+    if (!hasMonthTemplate || !hasLaneTemplate || !hasItemTemplate) {
       const missing = [];
       if (!hasMonthTemplate) missing.push('month-template');
       if (!hasLaneTemplate) missing.push('lane-template');
-      if (!hasTaskTemplate) missing.push('task-template');
+      if (!hasItemTemplate) missing.push('item-template');
 
       throw new Error(
         `Template is missing required elements: ${missing.join(', ')}. ` +
@@ -97,7 +97,7 @@ window.timelineGenerator = {
     // Extract template elements (will throw if not found)
     const monthTemplate = this.extractTemplate(template, 'month-template');
     const laneTemplate = this.extractTemplate(template, 'lane-template');
-    const taskTemplate = this.extractTemplate(template, 'task-template');
+    const itemTemplate = this.extractTemplate(template, 'item-template');
 
     const monthLabelY = 90;
     const gridTop = top - 20;
@@ -156,7 +156,7 @@ window.timelineGenerator = {
       laneElement = laneElement.replace(/id="lane-template"/, '');
       laneBlocks += laneElement;
 
-      // Generate tasks for this lane
+      // Generate items for this lane
       const laneItems = laneMap.get(laneName).sort((a, b) => a.due - b.due);
       laneItems.forEach((it, idx) => {
         const mi = monthIndexForDate(it.due);
@@ -164,19 +164,26 @@ window.timelineGenerator = {
         const cx = left + clampedMi * monthWidth + monthWidth * 0.5;
         const cy = laneY + 10 + idx * 18;
 
-        const taskValues = {
-          TASK_X: cx,
-          TASK_Y: cy,
+        // Start with layout placeholders
+        const itemValues = {
+          ITEM_X: cx,
+          ITEM_Y: cy,
           TEXT_X: cx + 12,
-          TEXT_Y: cy + 4,
-          TASK_ID: this.escapeXml(it.id || ""),
-          TASK_TITLE: this.escapeXml(it.title || "")
+          TEXT_Y: cy + 4
         };
 
-        let taskElement = this.replacePlaceholders(taskTemplate, taskValues);
-        taskElement = taskElement.replace(/style="display:\s*none;?"/, '');
-        taskElement = taskElement.replace(/id="task-template"/, '');
-        laneBlocks += taskElement;
+        // Dynamically add data placeholders from all item properties
+        for (const [key, value] of Object.entries(it)) {
+          if (key !== 'due') { // Skip due date as it's used for positioning
+            const placeholderName = `ITEM_${key.toUpperCase()}`;
+            itemValues[placeholderName] = this.escapeXml(value || "");
+          }
+        }
+
+        let itemElement = this.replacePlaceholders(itemTemplate, itemValues);
+        itemElement = itemElement.replace(/style="display:\s*none;?"/, '');
+        itemElement = itemElement.replace(/id="item-template"/, '');
+        laneBlocks += itemElement;
       });
     });
 
@@ -193,7 +200,7 @@ window.timelineGenerator = {
     processedTemplate = processedTemplate
       .replace(/<g id="month-template"[^>]*>[\s\S]*?<\/g>/, '')
       .replace(/<g id="lane-template"[^>]*>[\s\S]*?<\/g>/, '')
-      .replace(/<g id="task-template"[^>]*>[\s\S]*?<\/g>/, '');
+      .replace(/<g id="item-template"[^>]*>[\s\S]*?<\/g>/, '');
 
     // Add width and height attributes to the SVG element
     processedTemplate = processedTemplate.replace(