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

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "xer-mcp"
-version = "0.1.0"
+version = "0.2.0"
 description = "MCP server for querying Primavera P6 XER schedule data"
 readme = "README.md"
 requires-python = ">=3.14"