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/data-model.md

@@ -54,6 +54,7 @@ A unit of work in the schedule.
 | task_code | string | Yes | User-visible activity code |
 | task_name | string | Yes | Activity description |
 | task_type | enum | Yes | TT_Task, TT_Mile, TT_LOE, TT_WBS, TT_Rsrc |
+| milestone_type | enum | No | 'start' for start milestones, 'finish' for finish milestones, null for non-milestones |
 | target_start_date | datetime | No | Planned start |
 | target_end_date | datetime | No | Planned finish |
 | early_start_date | datetime | No | Calculated early start (for driving computation) |