Release 0.9.1: fix CLI NoneType when running dsagent without --model

[nmlemus]

Fixes #47

Changes

• CLI: Resolve effective model with get_default_model(explicit=args.model) before validate_configuration() in repl.py, run.py, and cli.py so dsagent / dsagent chat / dsagent run no longer raise 'NoneType' object has no attribute 'lower' when run without --model or config.
• Validation: validate_configuration() now guards against None/empty model and raises a clear ConfigurationError instead of AttributeError.
• Workflow: Add .cursor/rules/workflow-branch-pr.mdc and update AGENTS.md so changes are never committed directly to main (issue → branch → fix → test → PR).
• Version: Bump to 0.9.1.
• Test: ConversationalAgentConfig default model is None (resolved at runtime).

Made with Cursor