feat: Enhance chat command with multiple output formats and improve help documentation

- Updated the chat command to support new output formats: text, markdown, json, and compact.
- Modified the agent configuration to include output format settings.
- Enhanced the command line interface to handle new format options and provide detailed usage instructions.
- Improved the message printing logic in SimpleChatSession to format output based on the selected format.
- Added JSON and Markdown formatting for session metrics and messages.
- Updated help documentation to reflect changes in command usage and available options.

src/cli/handlers/agent.cc

@@ -73,7 +73,7 @@ constexpr absl::string_view kUsage =
   "  --ai_model=<name>       Model name (e.g., qwen2.5-coder:7b for Ollama)\n"
   "  --ollama_host=<url>     Ollama server URL (default: http://localhost:11434)\n"
   "  --gemini_api_key=<key>  Gemini API key (or set GEMINI_API_KEY env var)\n"
-  "  --format=<type>         Output format: json | table | yaml\n"
+  "  --format=<type>         Output format: text | markdown | json | compact\n"
   "\n"
   "For more details, see: docs/simple_chat_input_methods.md";