feat: implement XER MCP Server with 9 schedule query tools

Implement complete MCP server for parsing Primavera P6 XER files and
exposing schedule data through MCP tools. All 4 user stories complete.

Tools implemented:
- load_xer: Parse XER files into SQLite database
- list_activities: Query activities with pagination and filtering
- get_activity: Get activity details by ID
- list_relationships: Query activity dependencies
- get_predecessors/get_successors: Query activity relationships
- get_project_summary: Project overview with counts
- list_milestones: Query milestone activities
- get_critical_path: Query driving path activities

Features:
- Tab-delimited XER format parsing with pluggable table handlers
- In-memory SQLite database for fast queries
- Pagination with 100-item default limit
- Multi-project file support with project selection
- ISO8601 date formatting
- NO_FILE_LOADED error handling for all query tools

Test coverage: 81 tests (contract, integration, unit)

tests/contract/test_list_relationships.py

@@ -0,0 +1,78 @@
+"""Contract tests for list_relationships MCP tool."""
+
+from pathlib import Path
+
+import pytest
+
+from xer_mcp.db import db
+
+
+@pytest.fixture(autouse=True)
+def setup_db():
+    """Initialize and clear database for each test."""
+    db.initialize()
+    yield
+    db.clear()
+
+
+class TestListRelationshipsContract:
+    """Contract tests verifying list_relationships tool interface."""
+
+    async def test_list_relationships_returns_paginated_results(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """list_relationships returns relationships with pagination metadata."""
+        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 "pagination" in result
+        assert len(result["relationships"]) == 5
+        assert result["pagination"]["total_count"] == 5
+
+    async def test_list_relationships_with_pagination(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """list_relationships respects limit and offset parameters."""
+        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(limit=2, offset=0)
+
+        assert len(result["relationships"]) == 2
+        assert result["pagination"]["has_more"] is True
+
+    async def test_list_relationships_includes_expected_fields(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """list_relationships returns relationships with all expected fields."""
+        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()
+
+        rel = result["relationships"][0]
+        assert "task_pred_id" in rel
+        assert "task_id" in rel
+        assert "pred_task_id" in rel
+        assert "pred_type" in rel
+        assert "lag_hr_cnt" in rel
+
+    async def test_list_relationships_no_file_loaded_returns_error(self) -> None:
+        """list_relationships without loaded file returns NO_FILE_LOADED error."""
+        from xer_mcp.server import set_file_loaded
+        from xer_mcp.tools.list_relationships import list_relationships
+
+        set_file_loaded(False)
+        result = await list_relationships()
+
+        assert "error" in result
+        assert result["error"]["code"] == "NO_FILE_LOADED"