feat: add custom placeholder mapping for non-standard templates

Added optional placeholderMapping to project.json for templates that
don't follow the standard ITEM_{PROPERTY} convention.

Features:
- Override default placeholder names per field
- Useful for legacy templates or migration from other tools
- Fallback to ITEM_{PROPERTY} convention when not specified

Example usage:
{
  "fieldMapping": {
    "id": "ID",
    "title": "Title"
  },
  "placeholderMapping": {
    "id": "TASK_ID",     // Use {{TASK_ID}} instead of {{ITEM_ID}}
    "title": "TASK_NAME" // Use {{TASK_NAME}} instead of {{ITEM_TITLE}}
  }
}

Changes:
- generator.js: Check for cfg.placeholderMapping before using default
- Added 2 new tests for custom mapping behavior
- Updated TEMPLATE_V2_GUIDE.md with documentation and examples

All 58 tests passing.

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

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

TEMPLATE_V2_GUIDE.md

@@ -113,6 +113,32 @@ The following placeholders become available:
 
 **Note:** The `due` field is used for positioning and is not available as a placeholder (use `{{MONTH_LABEL}}` for date display).
 
+**Custom Placeholder Mapping:**
+
+If your template uses non-standard placeholder names, you can override the default convention using `placeholderMapping` in project.json:
+
+```json
+{
+  "fieldMapping": {
+    "id": "ID",
+    "title": "Title",
+    "assignee": "Assignee"
+  },
+  "placeholderMapping": {
+    "id": "TASK_ID",       // Use {{TASK_ID}} instead of {{ITEM_ID}}
+    "title": "TASK_NAME",   // Use {{TASK_NAME}} instead of {{ITEM_TITLE}}
+    "assignee": "ASSIGNEE"  // Use {{ASSIGNEE}} instead of {{ITEM_ASSIGNEE}}
+  }
+}
+```
+
+This is useful when:
+- Working with existing templates that use different naming conventions
+- Migrating from other timeline tools
+- Maintaining compatibility with legacy templates
+
+Without `placeholderMapping`, the default `ITEM_{PROPERTY}` convention is used.
+
 ### Global Placeholders
 
 These appear in the main template body (not in template elements):