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_successors.py

@@ -0,0 +1,76 @@
+"""Contract tests for get_successors 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 TestGetSuccessorsContract:
+    """Contract tests verifying get_successors tool interface."""
+
+    async def test_get_successors_returns_list(self, sample_xer_single_project: Path) -> None:
+        """get_successors returns successor activities."""
+        from xer_mcp.tools.get_successors import get_successors
+        from xer_mcp.tools.load_xer import load_xer
+
+        await load_xer(file_path=str(sample_xer_single_project))
+
+        # A1010 (2002) has two successors: A1020 (2003) and A1030 (2004)
+        result = await get_successors(activity_id="2002")
+
+        assert "activity_id" in result
+        assert result["activity_id"] == "2002"
+        assert "successors" in result
+        assert len(result["successors"]) == 2
+
+    async def test_get_successors_includes_relationship_details(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """get_successors includes relationship type and lag."""
+        from xer_mcp.tools.get_successors import get_successors
+        from xer_mcp.tools.load_xer import load_xer
+
+        await load_xer(file_path=str(sample_xer_single_project))
+
+        result = await get_successors(activity_id="2001")
+
+        succ = result["successors"][0]
+        assert "relationship_type" in succ
+        assert "lag_hr_cnt" in succ
+        assert succ["relationship_type"] in ["FS", "SS", "FF", "SF"]
+
+    async def test_get_successors_empty_list_for_no_successors(
+        self, sample_xer_single_project: Path
+    ) -> None:
+        """get_successors returns empty list when no successors exist."""
+        from xer_mcp.tools.get_successors import get_successors
+        from xer_mcp.tools.load_xer import load_xer
+
+        await load_xer(file_path=str(sample_xer_single_project))
+
+        # A1040 (2005) has no successors
+        result = await get_successors(activity_id="2005")
+
+        assert "successors" in result
+        assert len(result["successors"]) == 0
+
+    async def test_get_successors_no_file_loaded_returns_error(self) -> None:
+        """get_successors without loaded file returns NO_FILE_LOADED error."""
+        from xer_mcp.server import set_file_loaded
+        from xer_mcp.tools.get_successors import get_successors
+
+        set_file_loaded(False)
+        result = await get_successors(activity_id="2002")
+
+        assert "error" in result
+        assert result["error"]["code"] == "NO_FILE_LOADED"