Update marimo notebook docs with lessons from workflow debugging

- Add rules: all imports in setup cell, cell output at top level,
  async cells need async def, return classes from model cells,
  use python-dotenv for .env loading
- Add marimo check validation step to AGENTS.md and notebook-patterns.md
- Add "always create new workflow" rule to AGENTS.md
- Add new doc sections: Cell Output Must Be at the Top Level,
  Async Cells, Cells That Define Classes, Fixing _unparsable_cell,
  Checking Notebooks Before Running
- Update all code examples to follow new import/output rules
- Update workflows/lib/llm.py for mirascope v2 API

AGENTS.md

@@ -53,6 +53,10 @@ Also create a notebook when the user asks to "create a workflow", "write a workf
 
 If you're unsure whether a question is simple enough to answer directly or needs a notebook, **ask the user**.
 
+### Always create a new workflow
+
+When the user requests a workflow, **always create a new notebook file**. Do **not** modify or re-run an existing workflow unless the user explicitly asks you to (e.g., "update workflow 001", "fix the sentiment notebook", "re-run the existing analysis"). Each new request gets its own sequentially numbered file — even if it covers a similar topic to an earlier workflow.
+
 ### File naming and location
 
 All notebooks go in the **`workflows/`** directory. Use a sequential number prefix so workflows stay ordered by creation:
@@ -87,6 +91,16 @@ Before writing any notebook, **always propose a plan first** and get the user's
 
 Only proceed to implementation after the user confirms the plan.
 
+### Validate before delivering
+
+After writing or editing a notebook, **always run `uvx marimo check`** to verify it has no structural errors (duplicate variables, undefined names, branch expressions, etc.):
+
+```bash
+uvx marimo check workflows/NNN_topic_scope.py
+```
+
+A clean check (no output, exit code 0) means the notebook is valid. Fix any errors before delivering the notebook to the user.
+
 ### Steps
 
 1. **Identify people** — Use ContactDB to resolve names/emails to `contact_id` values. For "me"/"my" questions, always start with `GET /api/contacts/me`.