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/contracts/mcp-tools.json

@@ -336,7 +336,11 @@
           "type": "string",
           "enum": ["FS", "SS", "FF", "SF"]
         },
-        "lag_hr_cnt": { "type": "number" }
+        "lag_hr_cnt": { "type": "number" },
+        "driving": {
+          "type": "boolean",
+          "description": "True if this relationship drives the successor's early start date"
+        }
       }
     },
     "RelatedActivity": {
@@ -349,7 +353,11 @@
           "type": "string",
           "enum": ["FS", "SS", "FF", "SF"]
         },
-        "lag_hr_cnt": { "type": "number" }
+        "lag_hr_cnt": { "type": "number" },
+        "driving": {
+          "type": "boolean",
+          "description": "True if this relationship drives the successor's early start date"
+        }
       }
     },
     "Pagination": {