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/get_critical_path.py

@@ -0,0 +1,36 @@
+"""get_critical_path MCP tool implementation."""
+
+from xer_mcp.db.queries import query_critical_path
+from xer_mcp.server import is_file_loaded
+
+
+async def get_critical_path() -> dict:
+    """Get all activities on the critical path.
+
+    Returns activities where driving_path_flag is set, ordered by target start date.
+    The critical path determines the minimum project duration.
+
+    Returns:
+        Dictionary with list of critical path activities, each containing:
+        - task_id: Activity ID
+        - task_code: Activity code
+        - task_name: Activity name
+        - task_type: Activity type (TT_Task, TT_Mile, etc.)
+        - target_start_date: Target start date
+        - target_end_date: Target end date
+        - total_float_hr_cnt: Total float in hours
+        - 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.",
+            }
+        }
+
+    critical_activities = query_critical_path()
+
+    return {
+        "critical_activities": critical_activities,
+    }