feat: add driving flag to relationship query responses

Add computed driving flag to all relationship queries (list_relationships,
get_predecessors, get_successors). A relationship is marked as driving when
the predecessor's early end date plus lag determines the successor's early
start date.

Changes:
- Add early_start_date and early_end_date columns to activities schema
- Parse early dates from TASK table in XER files
- Implement is_driving_relationship() helper with 24hr tolerance for
  calendar gaps
- Update all relationship queries to compute and return driving flag
- Add contract and unit tests for driving flag functionality
- Update spec, contracts, and documentation

specs/001-schedule-tools/quickstart.md

@@ -152,19 +152,23 @@ Use the get_predecessors tool with activity_id="A1050"
       "task_code": "A1000",
       "task_name": "Site Preparation",
       "relationship_type": "FS",
-      "lag_hr_cnt": 0
+      "lag_hr_cnt": 0,
+      "driving": true
     },
     {
       "task_id": "A1010",
       "task_code": "A1010",
       "task_name": "Permits Approved",
       "relationship_type": "FS",
-      "lag_hr_cnt": 8
+      "lag_hr_cnt": 8,
+      "driving": false
     }
   ]
 }
 ```
 
+The `driving` flag indicates which predecessor relationship constrains the successor's early start date. A driving relationship is one where the predecessor's completion (plus lag) determines when the successor can begin.
+
 ### 6. Get Project Summary
 
 ```