Add comprehensive Flex Query Web Service integration to enable retrieval of
account statements, trade confirmations, and historical data from Interactive
Brokers. Flex Queries are automatically saved for easy reuse.

Features:
- Execute Flex Queries with automatic retry and polling for results
- Auto-save executed queries with friendly names from API responses
- List all previously used Flex Queries
- Forget/delete saved queries by ID or name
- Optional XML parsing to JSON for easier data consumption
- Persistent storage of query metadata in user's home directory

New Components:
- FlexQueryClient: HTTP client for IB Flex Web Service API v3
  * sendRequest: Initiate query execution
  * getStatement: Retrieve query results with polling
  * executeQuery: High-level method combining both steps
  * parseStatement: Convert XML responses to JSON

- FlexQueryStorage: Persistent storage manager for query metadata
  * JSON-based storage in ~/.interactive-brokers-mcp/flex-queries.json
  * CRUD operations for saved queries
  * Query lookup by ID, IB query ID, or friendly name
  * Automatic timestamp tracking (created, last used)

Configuration:
- IB_FLEX_TOKEN: Environment variable for Flex Web Service token
- Queries auto-saved on first execution with name from API
- Optional friendly name override during execution

Testing:
- 17 tests for FlexQueryClient covering all API interactions
- 24 tests for FlexQueryStorage covering all storage operations
- 10 additional tests for tool handlers covering Flex Query tools
- All tests passing (120 passed, 1 skipped)

Documentation:
- Updated README with Flex Query configuration instructions
- Added links to IB documentation for setup
- Documented all three new MCP tools

Breaking Changes: None - purely additive feature