Add Jupyter support to brev open command

Summary

This PR extends the brev open command to support Jupyter notebooks by adding brev open <instance-id> jupyter. The implementation:

• Adds EditorJupyter constant and updates all validation logic to include "jupyter" as a valid editor type
• Implements openJupyter() function that starts port forwarding on port 8888 and opens browser to localhost:8888
• Extends OpenStore interface to include util.MakeWorkspaceWithMetaStore for port forwarding compatibility
• Updates help text, examples, and error messages to include Jupyter support
• Follows existing patterns from the brev notebook command for Jupyter integration

The implementation reuses existing port forwarding infrastructure and browser opening utilities, maintaining consistency with the current codebase architecture.

Review & Testing Checklist for Human

• Test with live instance: Verify brev open <instance-id> jupyter actually works with a real Brev workspace
• Test port forwarding timing: Check if 2-second delay is sufficient for port forwarding to establish before browser opens
• Test error scenarios: Verify behavior when port forwarding fails or Jupyter isn't running on the instance
• Verify interface compatibility: Ensure OpenStore interface extension doesn't break existing implementations
• Test browser opening: Confirm browser opening works on different platforms (especially headless environments)

Recommended Test Plan:

1. Start a Brev workspace with Jupyter installed
2. Run brev open <workspace> jupyter
3. Verify browser opens to localhost:8888 and Jupyter is accessible
4. Test with workspace that doesn't have Jupyter running
5. Test existing editors still work: brev open <workspace> code

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    OpenCmd["pkg/cmd/open/open.go<br/>Main open command logic"]:::major-edit
    PortForward["pkg/cmd/portforward/portforward.go<br/>Port forwarding utilities"]:::context
    Browser["github.com/pkg/browser<br/>Browser opening library"]:::context
    UtilStore["pkg/cmd/util/util.go<br/>MakeWorkspaceWithMetaStore interface"]:::context
    
    OpenCmd -->|"Uses RunPortforward()"| PortForward
    OpenCmd -->|"Uses OpenURL()"| Browser
    OpenCmd -->|"Extends interface with"| UtilStore
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

Notes

• The openJupyter() function blocks indefinitely with select {} following the pattern from the notebook command
• Port forwarding runs in a goroutine with error handling that only prints to terminal
• Interface extension was necessary to satisfy portforward.PortforwardStore requirements
• Implementation assumes Jupyter will be accessible on port 8888 within the workspace

Link to Devin run: https://app.devin.ai/sessions/fd9c4f3f8e0648e0ade8845094dbb8ef
Requested by: @theFong