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/integration.test.js

@@ -58,7 +58,7 @@ describe('Timeline Integration', () => {
       // Verify no template placeholders remain
       expect(viewer.innerHTML).not.toContain('{{MONTH_X}}')
       expect(viewer.innerHTML).not.toContain('{{LANE_Y}}')
-      expect(viewer.innerHTML).not.toContain('{{TASK_X}}')
+      expect(viewer.innerHTML).not.toContain('{{ITEM_X}}')
 
       // Verify download button enabled
       const downloadBtn = document.getElementById('downloadSvg')
@@ -257,9 +257,9 @@ describe('Timeline Integration', () => {
       expect(viewer).toBeTruthy()
     })
 
-    it('should reject malformed template-v2 (missing task-template)', async () => {
+    it('should reject malformed template-v2 (missing item-template)', async () => {
       const config = createSampleProject()
-      const malformedTemplate = createMalformedTemplate('task-template')
+      const malformedTemplate = createMalformedTemplate('item-template')
 
       mockFetch(malformedTemplate)
       mockFetch(createSampleCSV())