feat: add direct database access for scripts (v0.2.0)

Implement persistent SQLite database feature that allows scripts to
query schedule data directly via SQL after loading XER files through MCP.

Key changes:
- Extend load_xer with db_path parameter for persistent database
- Add get_database_info tool to retrieve database connection details
- Add schema introspection with tables, columns, primary/foreign keys
- Support WAL mode for concurrent read access
- Use atomic write pattern to prevent corruption

New features:
- db_path=None: in-memory database (default, backward compatible)
- db_path="": auto-generate path from XER filename (.sqlite extension)
- db_path="/path/to/db": explicit persistent database path

Response includes complete DatabaseInfo:
- db_path: absolute path (or :memory:)
- is_persistent: boolean
- source_file: loaded XER path
- loaded_at: ISO timestamp
- schema: tables with columns, primary keys, foreign keys, row counts

Closes: User Story 1, 2, 3 from 002-direct-db-access spec

src/xer_mcp/tools/get_database_info.py

@@ -0,0 +1,25 @@
+"""get_database_info MCP tool implementation."""
+
+from xer_mcp.db import db
+
+
+async def get_database_info() -> dict:
+    """Get information about the currently loaded database.
+
+    Returns connection details for direct SQL access including
+    database path, schema information, and metadata.
+
+    Returns:
+        Dictionary with database info or error if no database loaded
+    """
+    if not db.is_initialized:
+        return {
+            "error": {
+                "code": "NO_FILE_LOADED",
+                "message": "No XER file is loaded. Use the load_xer tool first.",
+            }
+        }
+
+    return {
+        "database": db.get_database_info(),
+    }