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/models/activity.py

@@ -0,0 +1,23 @@
+"""Activity data model."""
+
+from dataclasses import dataclass
+from datetime import datetime
+
+
+@dataclass
+class Activity:
+    """A unit of work in the schedule."""
+
+    task_id: str
+    proj_id: str
+    task_code: str
+    task_name: str
+    task_type: str
+    wbs_id: str | None = None
+    target_start_date: datetime | None = None
+    target_end_date: datetime | None = None
+    act_start_date: datetime | None = None
+    act_end_date: datetime | None = None
+    total_float_hr_cnt: float | None = None
+    driving_path_flag: bool = False
+    status_code: str | None = None