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_get_critical_path.py

@@ -0,0 +1,91 @@
+"""Contract tests for get_critical_path 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 TestGetCriticalPathContract:
+    """Contract tests verifying get_critical_path tool interface."""
+
+    async def test_get_critical_path_returns_critical_activities(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """get_critical_path returns activities with driving_path_flag set."""
+        from xer_mcp.tools.get_critical_path import get_critical_path
+        from xer_mcp.tools.load_xer import load_xer
+
+        await load_xer(file_path=str(sample_xer_single_project))
+
+        result = await get_critical_path()
+
+        assert "critical_activities" in result
+        assert len(result["critical_activities"]) == 4
+
+    async def test_get_critical_path_includes_expected_fields(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """get_critical_path returns activities with required fields."""
+        from xer_mcp.tools.get_critical_path import get_critical_path
+        from xer_mcp.tools.load_xer import load_xer
+
+        await load_xer(file_path=str(sample_xer_single_project))
+
+        result = await get_critical_path()
+
+        activity = result["critical_activities"][0]
+        assert "task_id" in activity
+        assert "task_code" in activity
+        assert "task_name" in activity
+        assert "target_start_date" in activity
+        assert "target_end_date" in activity
+        assert "total_float_hr_cnt" in activity
+
+    async def test_get_critical_path_ordered_by_date(self, sample_xer_single_project: Path) -> None:
+        """get_critical_path returns activities ordered by start date."""
+        from xer_mcp.tools.get_critical_path import get_critical_path
+        from xer_mcp.tools.load_xer import load_xer
+
+        await load_xer(file_path=str(sample_xer_single_project))
+
+        result = await get_critical_path()
+
+        activities = result["critical_activities"]
+        for i in range(len(activities) - 1):
+            assert activities[i]["target_start_date"] <= activities[i + 1]["target_start_date"]
+
+    async def test_get_critical_path_excludes_non_critical(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """get_critical_path excludes activities not on critical path."""
+        from xer_mcp.tools.get_critical_path import get_critical_path
+        from xer_mcp.tools.load_xer import load_xer
+
+        await load_xer(file_path=str(sample_xer_single_project))
+
+        result = await get_critical_path()
+
+        # A1020 "Foundation Work" has driving_path_flag = N
+        activity_names = [a["task_name"] for a in result["critical_activities"]]
+        assert "Foundation Work" not in activity_names
+
+    async def test_get_critical_path_no_file_loaded_returns_error(self) -> None:
+        """get_critical_path without loaded file returns NO_FILE_LOADED error."""
+        from xer_mcp.server import set_file_loaded
+        from xer_mcp.tools.get_critical_path import get_critical_path
+
+        set_file_loaded(False)
+        result = await get_critical_path()
+
+        assert "error" in result
+        assert result["error"]["code"] == "NO_FILE_LOADED"