Add tmux support to brev open command

[devin-ai-integration]

Add tmux support to brev open command

Summary

This PR extends the brev open command to support tmux as a new editor type alongside VSCode, Cursor, and Windsurf. Users can now run brev open <instance> tmux to connect to a persistent tmux session on their remote workspace.

Key features added:

• New EditorTmux constant and validation across all editor type checks
• openTmux() function with intelligent session management (reconnects to existing sessions or creates new ones)
• ensureTmuxInstalled() for automatic tmux installation on remote instances
• Session naming convention: brev-<workspace-identifier> for predictable, conflict-free sessions
• Interactive SSH execution for seamless tmux experience

Implementation follows existing patterns:

• Same validation and error handling structure as VSCode/Cursor/Windsurf
• Consistent help text and CLI interface updates
• Proper error wrapping and security annotations

Review & Testing Checklist for Human

• Security Review: Verify SSH command construction in openTmux() and ensureTmuxInstalled() is safe from injection attacks, especially with user-controlled sshAlias and path parameters
• End-to-end Testing: Test brev open <instance> tmux with a real workspace to verify session creation, reconnection, and working directory handling
• Installation Testing: Test tmux installation on instances without tmux, and verify graceful failure on non-Ubuntu systems or restricted environments
• Session Management: Verify that brev-<workspace> session naming works correctly and doesn't conflict with existing user tmux sessions
• Error Scenarios: Test failure cases like SSH connection issues, tmux installation failures, and session creation problems

Recommended test plan:

1. Test with fresh instance (no tmux) - should auto-install and create session
2. Test reconnection - should attach to existing brev-<workspace> session
3. Test with manual tmux sessions already running - should not conflict
4. Test error cases - network issues, permission problems, non-Ubuntu systems

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    CLI["brev open <instance> tmux"]
    OpenCmd["pkg/cmd/open/open.go<br/>runOpenCommand()"]
    Determine["determineEditorType()"]
    OpenByType["openEditorByType()"]
    OpenTmux["openTmux()"]:::major-edit
    EnsureTmux["ensureTmuxInstalled()"]:::major-edit
    SSH["Interactive SSH<br/>exec.Command()"]
    
    CLI --> OpenCmd
    OpenCmd --> Determine
    Determine --> OpenByType
    OpenByType --> OpenTmux
    OpenTmux --> EnsureTmux
    OpenTmux --> SSH
    
    Constants["Editor Constants<br/>EditorTmux = 'tmux'"]:::major-edit
    Validation["Validation Functions<br/>handleSetDefault()<br/>determineEditorType()"]:::major-edit
    HelpText["Help Text & Examples"]:::minor-edit
    
    OpenCmd --> Constants
    OpenCmd --> Validation
    OpenCmd --> HelpText
    
    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

Loading

Notes

Architectural differences from other editors:

• VSCode/Cursor/Windsurf use vscode-remote://ssh-remote+ URIs and local editor execution
• tmux uses direct interactive SSH execution (ssh -t <alias> 'tmux ...')
• No extension installation needed (unlike other editors)

Session management strategy:

• Session names: brev-<workspace-identifier> (predictable and discoverable)
• Automatic reconnection to existing sessions
• Working directory preservation for new sessions
• Sessions are lost on workspace restart (expected behavior)

Platform considerations:

• tmux installation assumes Ubuntu/Debian with apt-get
• Could fail on other distributions or restricted environments
• Error handling provides feedback but may not cover all edge cases

---

Link to Devin run: https://app.devin.ai/sessions/2735e7dbd59c4a90ae870a0e00f46668
Requested by: Alec Fong (@theFong)

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[theFong]

for the tmux session lets just call it "brev" no alias. That way if we change the name in the future we always connect to the same session.