refactor: update milestone/task status enums to new state machine values

Milestone: open/freeze/undergoing/completed/closed (was open/pending/deferred/progressing/closed)
Task: open/pending/undergoing/completed/closed (was open/pending/progressing/closed)

- Add MilestoneStatusEnum to schemas with typed validation
- Add started_at field to Milestone model
- Update all router/CLI references from progressing->undergoing
- Add completed status handling in task transition logic

app/models/task.py

@@ -7,7 +7,8 @@ import enum
 class TaskStatus(str, enum.Enum):
     OPEN = "open"
     PENDING = "pending"
-    PROGRESSING = "progressing"
+    UNDERGOING = "undergoing"
+    COMPLETED = "completed"
     CLOSED = "closed"
 
 class TaskPriority(str, enum.Enum):