Bump app version to 1.2.9

Updated the APP_VERSION constant in main.py from 1.2.5 to 1.2.9 to reflect the latest release.

Enhance model selection and add command auto-completion

Added advanced model selection options, including browsing by task category, capabilities, and groups using OpenRouter's enhanced API. Implemented dynamic detection of model features such as reasoning/thinking mode and pricing info. Integrated command auto-completion with descriptions using prompt_toolkit for improved CLI usability. Updated README with detailed feature breakdowns, new usage instructions, and improved documentation structure.