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_activity.py

@@ -0,0 +1,34 @@
+"""get_activity MCP tool implementation."""
+
+from xer_mcp.db.queries import get_activity_by_id
+from xer_mcp.server import is_file_loaded
+
+
+async def get_activity(activity_id: str) -> dict:
+    """Get detailed information for a specific activity by ID.
+
+    Args:
+        activity_id: The task_id of the activity
+
+    Returns:
+        Dictionary with complete activity details or error
+    """
+    if not is_file_loaded():
+        return {
+            "error": {
+                "code": "NO_FILE_LOADED",
+                "message": "No XER file is loaded. Use the load_xer tool first.",
+            }
+        }
+
+    activity = get_activity_by_id(activity_id)
+
+    if activity is None:
+        return {
+            "error": {
+                "code": "ACTIVITY_NOT_FOUND",
+                "message": f"Activity not found: {activity_id}",
+            }
+        }
+
+    return activity