Upgrade llm_gateway to latest version

[managerbot-app]

Summary

• Upgrade llm_gateway from the old refactor branch (0.3.0) to the latest confirmed tag, v0.8.0.
• Migrate provider keys/model handling for the 0.6+ API (model: per stream call; canonical provider names including Groq).
• Update prompt initialization, AssistantMessage test fixtures, normalized usage handling, and renamed OpenAI/Anthropic client constants.
• Remove duplicated bash/read/write/edit tool implementations and use llm_gateway's built-in agent tools.

Tests

• ruby -I/tmp/llm_gateway/lib -I/tmp/rubister -rllm_gateway test/sessions/base_session_manager_test.rb
• ruby -I/tmp/llm_gateway/lib -I/tmp/rubister -rllm_gateway test/sessions/file_session_manager_test.rb
• ruby -I/tmp/llm_gateway/lib -I/tmp/rubister -rllm_gateway test/agents/agent_tools_test.rb
• ruby -I/tmp/llm_gateway/lib -I/tmp/rubister -rllm_gateway test/telegram/poller_test.rb
• ruby -I/tmp/llm_gateway/lib -I/tmp/rubister -rllm_gateway -e 'Dir["test/**/*_test.rb"].sort.each { |f| require_relative f }'
• git diff --check

Full Bundler install/test could not run in this container because rubister's Gemfile specifies Ruby 4.0.1 while the available Ruby is 3.3.11.