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/contracts/mcp-tools.json

@@ -301,7 +301,8 @@
         "target_start_date": { "type": "string", "format": "date-time" },
         "target_end_date": { "type": "string", "format": "date-time" },
         "status_code": { "type": "string" },
-        "driving_path_flag": { "type": "boolean" }
+        "driving_path_flag": { "type": "boolean" },
+        "milestone_type": { "type": "string", "enum": ["start", "finish", null], "description": "Type of milestone: 'start' for start milestones, 'finish' for finish milestones, null for non-milestones" }
       }
     },
     "ActivityDetail": {