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

tests/unit/test_db_queries.py

@@ -109,3 +109,68 @@ class TestActivityQueries:
         activity = get_activity_by_id("nonexistent")
 
         assert activity is None
+
+
+class TestDrivingRelationship:
+    """Tests for driving relationship computation."""
+
+    def test_is_driving_relationship_fs_driving(self) -> None:
+        """FS relationship is driving when pred_end + lag = succ_start."""
+        from xer_mcp.db.queries import is_driving_relationship
+
+        # Pred ends at 2026-01-08T15:00, succ starts at 2026-01-09T07:00
+        # With 0 lag and overnight gap, this is driving
+        result = is_driving_relationship(
+            pred_early_end="2026-01-08T15:00:00",
+            succ_early_start="2026-01-09T07:00:00",
+            lag_hours=0.0,
+            pred_type="FS",
+        )
+        assert result is True
+
+    def test_is_driving_relationship_fs_not_driving(self) -> None:
+        """FS relationship is not driving when there's float."""
+        from xer_mcp.db.queries import is_driving_relationship
+
+        # Pred ends much earlier than succ starts (has float)
+        result = is_driving_relationship(
+            pred_early_end="2026-01-01T15:00:00",
+            succ_early_start="2026-01-10T07:00:00",
+            lag_hours=0.0,
+            pred_type="FS",
+        )
+        assert result is False
+
+    def test_is_driving_relationship_with_lag(self) -> None:
+        """FS relationship with lag is driving when pred_end + lag = succ_start."""
+        from xer_mcp.db.queries import is_driving_relationship
+
+        # Pred ends at 2026-01-08T15:00, 16hr lag, succ starts at 2026-01-09T07:00
+        # 15:00 + 16hrs = next day 07:00 (exactly matches)
+        result = is_driving_relationship(
+            pred_early_end="2026-01-08T15:00:00",
+            succ_early_start="2026-01-09T07:00:00",
+            lag_hours=16.0,
+            pred_type="FS",
+        )
+        assert result is True
+
+    def test_is_driving_relationship_missing_dates(self) -> None:
+        """Relationship is not driving when dates are missing."""
+        from xer_mcp.db.queries import is_driving_relationship
+
+        result = is_driving_relationship(
+            pred_early_end=None,
+            succ_early_start="2026-01-09T07:00:00",
+            lag_hours=0.0,
+            pred_type="FS",
+        )
+        assert result is False
+
+        result = is_driving_relationship(
+            pred_early_end="2026-01-08T15:00:00",
+            succ_early_start=None,
+            lag_hours=0.0,
+            pred_type="FS",
+        )
+        assert result is False