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')

specs/001-schedule-tools/quickstart.md

@@ -226,6 +226,34 @@ Use the get_critical_path tool
 Use the list_milestones tool
 ```
 
+**Response**:
+```json
+{
+  "milestones": [
+    {
+      "task_id": "M001",
+      "task_code": "MS-START",
+      "task_name": "Project Start",
+      "target_start_date": "2026-01-15T08:00:00",
+      "target_end_date": "2026-01-15T08:00:00",
+      "status_code": "TK_Complete",
+      "milestone_type": "start"
+    },
+    {
+      "task_id": "M025",
+      "task_code": "MS-END",
+      "task_name": "Project Complete",
+      "target_start_date": "2026-06-30T17:00:00",
+      "target_end_date": "2026-06-30T17:00:00",
+      "status_code": "TK_NotStart",
+      "milestone_type": "finish"
+    }
+  ]
+}
+```
+
+The `milestone_type` field indicates whether the milestone is a Start Milestone (`"start"`) or a Finish Milestone (`"finish"`).
+
 ## Pagination
 
 All list operations support pagination: