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)

src/xer_mcp/tools/list_milestones.py

@@ -0,0 +1,31 @@
+"""list_milestones MCP tool implementation."""
+
+from xer_mcp.db.queries import query_milestones
+from xer_mcp.server import is_file_loaded
+
+
+async def list_milestones() -> dict:
+    """List all milestone activities in the loaded project.
+
+    Returns:
+        Dictionary with list of milestones, each containing:
+        - task_id: Activity ID
+        - task_code: Activity code
+        - task_name: Activity name
+        - target_start_date: Target start date
+        - target_end_date: Target end date
+        - status_code: Activity status
+    """
+    if not is_file_loaded():
+        return {
+            "error": {
+                "code": "NO_FILE_LOADED",
+                "message": "No XER file is loaded. Use the load_xer tool first.",
+            }
+        }
+
+    milestones = query_milestones()
+
+    return {
+        "milestones": milestones,
+    }