test and adapt `marimo` support within positron (experimental)

[rodrigosf672]

Screenshot from dev build. This is just a proof of concept, and any feedback would be highly appreciated!

Screen.Recording.2025-12-12.at.15.25.17.mov

---

Summary

Add safe, viewer-pane support for Marimo notebooks in Positron using CLI delegation, without changing Positron’s execution model.

Problem

• Users encounter Marimo notebooks (.py with import marimo)
• Positron cannot open them easily
• Users must switch to VS Code or the terminal
  • Depending on how savvy the user is, they may be able to remove the token manually and run the Marimo (already in the current release).
  • However, this is cumbersome and it'd be better to have a straightforward, pleasant experience.
• Notebook interactivity is a known gap

Proposal (Phase 1)

Provide open-only support via the Marimo CLI:

P1
Already partially implemented, although extra steps are needed to skip the step of Cmd+Click the link in order to open it in the Viewer pane.

• Detect Marimo notebooks
• Open Marimo editor UI in the viewer pane
  (marimo edit <file>)

P2
Easy win but not essential, just to enable user to install marimo with a button instead of pip install.

• Environment-scoped install guidance (pip/conda)

P3
That could be a nice addition to enable users to convert between .ipynb and .py files.

• Non-destructive conversion Jupyter ↔ Marimo
  (marimo convert ...)

P4
This would perhaps be the most challenging part, but perhaps high value.

• Run Marimo app (marimo run <file>)

Non-goals (Phase 1)

• Embedded/runtime Marimo execution
• Kernel-managed Marimo
• Native Marimo editor (non-CLI)
• Widget fixes
• VS Code Marimo extension support

UX & Safety

• Open ≠ Execute (no auto-run)
• Viewer-pane only (also opens in the Editor)
• No file mutation
• Explicit user intent
• Surface CLI errors verbatim

Acceptance Criteria

• Marimo notebooks open reliably in the viewer pane
• No execution regressions
• No file modification
• Clear user messaging about open-only behavior (run is P3)

[juliasilge]

Our main options today are to either handle Marimo via positron-run-app or to make a PR to the Marimo VS Code extension so that it opens in the Viewer pane when in Positron. I lean toward the latter.