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/generator.test.js

@@ -66,12 +66,12 @@ describe('Timeline Generator', () => {
       }).toThrow('Template is missing required elements: lane-template')
     })
 
-    it('should throw error when template is missing task-template', () => {
-      const malformedTemplate = createMalformedTemplate('task-template')
+    it('should throw error when template is missing item-template', () => {
+      const malformedTemplate = createMalformedTemplate('item-template')
 
       expect(() => {
         timelineGenerator.generate(items, config, malformedTemplate)
-      }).toThrow('Template is missing required elements: task-template')
+      }).toThrow('Template is missing required elements: item-template')
     })
 
     it('should validate template with proper error message', () => {
@@ -179,8 +179,8 @@ describe('Timeline Generator', () => {
 
       expect(result).not.toContain('{{MONTH_X}}')
       expect(result).not.toContain('{{LANE_Y}}')
-      expect(result).not.toContain('{{TASK_X}}')
-      expect(result).not.toContain('{{TASK_TITLE}}')
+      expect(result).not.toContain('{{ITEM_X}}')
+      expect(result).not.toContain('{{ITEM_TITLE}}')
     })
 
     it('should contain task data in generated SVG', () => {
@@ -248,7 +248,7 @@ describe('Timeline Generator', () => {
 
       expect(result).not.toContain('id="month-template"')
       expect(result).not.toContain('id="lane-template"')
-      expect(result).not.toContain('id="task-template"')
+      expect(result).not.toContain('id="item-template"')
       expect(result).not.toContain('style="display:none"')
     })