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/db/schema.py

@@ -0,0 +1,77 @@
+"""SQLite database schema for XER MCP Server."""
+
+SCHEMA_SQL = """
+-- Projects
+CREATE TABLE IF NOT EXISTS projects (
+    proj_id TEXT PRIMARY KEY,
+    proj_short_name TEXT NOT NULL,
+    plan_start_date TEXT,
+    plan_end_date TEXT,
+    loaded_at TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+-- Activities
+CREATE TABLE IF NOT EXISTS activities (
+    task_id TEXT PRIMARY KEY,
+    proj_id TEXT NOT NULL,
+    wbs_id TEXT,
+    task_code TEXT NOT NULL,
+    task_name TEXT NOT NULL,
+    task_type TEXT NOT NULL,
+    target_start_date TEXT,
+    target_end_date TEXT,
+    act_start_date TEXT,
+    act_end_date TEXT,
+    total_float_hr_cnt REAL,
+    driving_path_flag INTEGER DEFAULT 0,
+    status_code TEXT,
+    FOREIGN KEY (proj_id) REFERENCES projects(proj_id)
+);
+
+-- Relationships
+CREATE TABLE IF NOT EXISTS relationships (
+    task_pred_id TEXT PRIMARY KEY,
+    task_id TEXT NOT NULL,
+    pred_task_id TEXT NOT NULL,
+    pred_type TEXT NOT NULL,
+    lag_hr_cnt REAL DEFAULT 0,
+    FOREIGN KEY (task_id) REFERENCES activities(task_id),
+    FOREIGN KEY (pred_task_id) REFERENCES activities(task_id)
+);
+
+-- WBS (Work Breakdown Structure)
+CREATE TABLE IF NOT EXISTS wbs (
+    wbs_id TEXT PRIMARY KEY,
+    proj_id TEXT NOT NULL,
+    parent_wbs_id TEXT,
+    wbs_short_name TEXT NOT NULL,
+    wbs_name TEXT,
+    FOREIGN KEY (proj_id) REFERENCES projects(proj_id),
+    FOREIGN KEY (parent_wbs_id) REFERENCES wbs(wbs_id)
+);
+
+-- Calendars (internal use only)
+CREATE TABLE IF NOT EXISTS calendars (
+    clndr_id TEXT PRIMARY KEY,
+    clndr_name TEXT NOT NULL,
+    day_hr_cnt REAL,
+    week_hr_cnt REAL
+);
+
+-- Indexes for performance
+CREATE INDEX IF NOT EXISTS idx_activities_proj ON activities(proj_id);
+CREATE INDEX IF NOT EXISTS idx_activities_wbs ON activities(wbs_id);
+CREATE INDEX IF NOT EXISTS idx_activities_type ON activities(task_type);
+CREATE INDEX IF NOT EXISTS idx_activities_critical ON activities(driving_path_flag)
+    WHERE driving_path_flag = 1;
+CREATE INDEX IF NOT EXISTS idx_activities_dates ON activities(target_start_date, target_end_date);
+CREATE INDEX IF NOT EXISTS idx_relationships_task ON relationships(task_id);
+CREATE INDEX IF NOT EXISTS idx_relationships_pred ON relationships(pred_task_id);
+CREATE INDEX IF NOT EXISTS idx_wbs_parent ON wbs(parent_wbs_id);
+CREATE INDEX IF NOT EXISTS idx_wbs_proj ON wbs(proj_id);
+"""
+
+
+def get_schema() -> str:
+    """Return the complete SQLite schema."""
+    return SCHEMA_SQL