chore: consolidate install, fix CLI portability, normalize LLM responses

- Point requirements.txt to pyproject.toml as single source of truth
- Resolve welcome.txt path relative to module for CLI portability
- Include cli/static files in package build
- Extract shared normalize_content for OpenAI Responses API and
  Gemini 3 list-format responses into base_client.py
- Update README install and CLI usage instructions

pyproject.toml

@@ -37,3 +37,6 @@ tradingagents = "cli.main:app"
 
 [tool.setuptools.packages.find]
 include = ["tradingagents*", "cli*"]
+
+[tool.setuptools.package-data]
+cli = ["static/*"]