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/parser/table_handlers/projwbs.py

@@ -0,0 +1,26 @@
+"""PROJWBS table handler."""
+
+from xer_mcp.parser.table_handlers.base import TableHandler
+
+
+class ProjwbsHandler(TableHandler):
+    """Handler for PROJWBS (WBS) table in XER files."""
+
+    @property
+    def table_name(self) -> str:
+        return "PROJWBS"
+
+    def parse_row(self, fields: list[str], values: list[str]) -> dict | None:
+        """Parse a PROJWBS row."""
+        if len(values) < len(fields):
+            values = values + [""] * (len(fields) - len(values))
+
+        data = dict(zip(fields, values, strict=False))
+
+        return {
+            "wbs_id": data.get("wbs_id", ""),
+            "proj_id": data.get("proj_id", ""),
+            "parent_wbs_id": data.get("parent_wbs_id", ""),
+            "wbs_short_name": data.get("wbs_short_name", ""),
+            "wbs_name": data.get("wbs_name") or None,
+        }