feat: add milestone_type field to distinguish start/finish milestones

Add milestone_type field to milestone queries that indicates whether
a milestone is a start milestone ('start') or finish milestone ('finish').

Changes:
- Add milestone_type column to activities table schema
- Parse milestone_type from XER TASK table (MS_Start/MS_Finish)
- Include milestone_type in list_milestones response
- Update contract tests for milestone_type field
- Update specs, contracts, and documentation

The milestone_type is determined by:
1. Explicit milestone_type field in XER (MS_Start -> 'start', MS_Finish -> 'finish')
2. Derived from task_type (TT_Mile -> 'start', TT_FinMile -> 'finish')

src/xer_mcp/tools/list_milestones.py

@@ -7,6 +7,8 @@ from xer_mcp.server import is_file_loaded
 async def list_milestones() -> dict:
     """List all milestone activities in the loaded project.
 
+    Returns both Start Milestones and Finish Milestones with their milestone_type.
+
     Returns:
         Dictionary with list of milestones, each containing:
         - task_id: Activity ID
@@ -15,6 +17,7 @@ async def list_milestones() -> dict:
         - target_start_date: Target start date
         - target_end_date: Target end date
         - status_code: Activity status
+        - milestone_type: 'start' for start milestones, 'finish' for finish milestones
     """
     if not is_file_loaded():
         return {