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>

TEMPLATE_V2_GUIDE.md

@@ -12,7 +12,7 @@ A valid template-v2.svg file must contain:
 2. **`<defs>` section** containing three required template elements:
    - `<g id="month-template">` - Defines how each month column is rendered
    - `<g id="lane-template">` - Defines how each lane (epic/swimlane) is rendered
-   - `<g id="task-template">` - Defines how each task item is rendered
+   - `<g id="item-template">` - Defines how each task item is rendered
 3. **Main content area** with `{{MONTHS}}` and `{{LANES}}` placeholders
 4. **Optional styling** (gradients, filters, patterns, etc.)
 
@@ -37,11 +37,11 @@ A valid template-v2.svg file must contain:
             font-family="Arial" font-size="14" font-weight="bold" fill="#212121">{{LANE_NAME}}</text>
     </g>
 
-    <!-- Task template -->
-    <g id="task-template" style="display:none">
-      <circle cx="{{TASK_X}}" cy="{{TASK_Y}}" r="6" fill="#1976D2"/>
+    <!-- Item template -->
+    <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">{{TASK_ID}} {{TASK_TITLE}}</text>
+            font-family="Arial" font-size="11" fill="#424242">{{ITEM_ID}} {{ITEM_TITLE}}</text>
     </g>
   </defs>
 
@@ -80,16 +80,38 @@ A valid template-v2.svg file must contain:
 | `{{LABEL_Y}}` | Y position for lane label | 140 |
 | `{{LANE_NAME}}` | Lane name text (XML-escaped) | "Development" |
 
-### Task Template Placeholders
+### Item Template Placeholders
 
 | Placeholder | Description | Example Value |
 |------------|-------------|---------------|
-| `{{TASK_X}}` | X position of task marker | 400 |
-| `{{TASK_Y}}` | Y position of task marker | 150 |
-| `{{TEXT_X}}` | X position for task text | 412 |
-| `{{TEXT_Y}}` | Y position for task text | 154 |
-| `{{TASK_ID}}` | Task ID (XML-escaped) | "T-123" |
-| `{{TASK_TITLE}}` | Task title (XML-escaped) | "Implement feature" |
+| `{{ITEM_X}}` | X position of item marker | 400 |
+| `{{ITEM_Y}}` | Y position of item marker | 150 |
+| `{{TEXT_X}}` | X position for item text | 412 |
+| `{{TEXT_Y}}` | Y position for item text | 154 |
+| `{{ITEM_ID}}` | Item ID (XML-escaped) | "T-123" |
+| `{{ITEM_TITLE}}` | Item title (XML-escaped) | "Implement feature" |
+
+**Dynamic Data Placeholders:**
+
+The generator automatically creates placeholders for **all properties** in your CSV data using the naming convention: `ITEM_{PROPERTY_UPPERCASE}`.
+
+For example, if your `fieldMapping` includes:
+```json
+{
+  "id": "ID",
+  "title": "Title",
+  "assignee": "Assignee",
+  "priority": "Priority"
+}
+```
+
+The following placeholders become available:
+- `{{ITEM_ID}}` - from the `id` field
+- `{{ITEM_TITLE}}` - from the `title` field
+- `{{ITEM_ASSIGNEE}}` - from the `assignee` field
+- `{{ITEM_PRIORITY}}` - from the `priority` field
+
+**Note:** The `due` field is used for positioning and is not available as a placeholder (use `{{MONTH_LABEL}}` for date display).
 
 ### Global Placeholders
 
@@ -107,7 +129,7 @@ These appear in the main template body (not in template elements):
 1. Open template-v2.svg in Inkscape
 2. Locate template elements in the Layers panel (inside `<defs>`)
 3. Edit shapes, colors, fonts, etc. as needed
-4. **Important**: Keep `id="month-template"`, `id="lane-template"`, `id="task-template"` unchanged
+4. **Important**: Keep `id="month-template"`, `id="lane-template"`, `id="item-template"` unchanged
 5. **Important**: Keep `{{PLACEHOLDER}}` text exactly as is - these are replaced at runtime
 6. Save file (keep SVG format, avoid Inkscape-specific extensions)
 
@@ -190,8 +212,8 @@ Use SVG filters:
     </feMerge>
   </filter>
 
-  <g id="task-template" style="display:none">
-    <circle cx="{{TASK_X}}" cy="{{TASK_Y}}" r="6" fill="#1976D2" filter="url(#dropShadow)"/>
+  <g id="item-template" style="display:none">
+    <circle cx="{{ITEM_X}}" cy="{{ITEM_Y}}" r="6" fill="#1976D2" filter="url(#dropShadow)"/>
     ...
   </g>
 </defs>
@@ -229,7 +251,7 @@ To change the overall layout, you would need to modify generator.js. Templates c
 - Ensure template has all three required elements in `<defs>`:
   - `<g id="month-template">`
   - `<g id="lane-template">`
-  - `<g id="task-template">`
+  - `<g id="item-template">`
 - Check that IDs are exactly as shown (case-sensitive)
 - Verify elements are inside `<defs>` section