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/contract/test_list_relationships.py

@@ -76,3 +76,21 @@ class TestListRelationshipsContract:
 
         assert "error" in result
         assert result["error"]["code"] == "NO_FILE_LOADED"
+
+    async def test_list_relationships_includes_driving_flag(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """list_relationships returns relationships with driving flag."""
+        from xer_mcp.tools.list_relationships import list_relationships
+        from xer_mcp.tools.load_xer import load_xer
+
+        await load_xer(file_path=str(sample_xer_single_project))
+
+        result = await list_relationships()
+
+        assert "relationships" in result
+        assert len(result["relationships"]) > 0
+        # All relationships should have a driving flag
+        for rel in result["relationships"]:
+            assert "driving" in rel
+            assert isinstance(rel["driving"], bool)