Replace 'workspace' and 'my-app' with 'instance' in CLI help strings

Summary

This PR updates user-facing CLI help text, examples, and messages to use consistent "instance" terminology instead of "workspace" and "my-app". The changes are purely cosmetic string replacements that improve terminology consistency across the CLI without affecting any functionality.

Files modified:

• pkg/cmd/delete/delete.go: Updated error message
• pkg/cmd/open/open.go: Updated command description, examples, and status messages
• pkg/cmd/recreate/recreate.go: Updated spinner status messages
• pkg/cmd/reset/reset.go: Updated spinner status messages
• pkg/cmd/shell/shell.go: Updated command description, examples, and error message
• pkg/cmd/stop/stop.go: Updated command description, flag description, and error message

Types of changes:

• Command short/long descriptions: "workspace" → "instance"
• Command examples: "my-app" → "instance", "workspace_id_or_name" → "instance_id_or_name"
• User-facing status/error messages: "workspace" → "instance"
• Flag descriptions: "stop all workspaces" → "stop all instances"

Review & Testing Checklist for Human

• Run CLI commands manually - Execute brev help, brev shell --help, brev open --help, etc. to verify help text displays correctly with "instance" terminology
• Search for missed references - Check if there are other locations where "workspace" or "my-app" should be updated to "instance"
• Verify example consistency - Ensure all command examples now use "instance" consistently and make semantic sense

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    CLI["CLI Root<br/>pkg/cmd/cmd.go"]:::context
    Shell["Shell Command<br/>pkg/cmd/shell/shell.go"]:::major-edit
    Open["Open Command<br/>pkg/cmd/open/open.go"]:::major-edit
    Stop["Stop Command<br/>pkg/cmd/stop/stop.go"]:::major-edit
    Delete["Delete Command<br/>pkg/cmd/delete/delete.go"]:::minor-edit
    Reset["Reset Command<br/>pkg/cmd/reset/reset.go"]:::minor-edit
    Recreate["Recreate Command<br/>pkg/cmd/recreate/recreate.go"]:::minor-edit

    CLI --> Shell
    CLI --> Open  
    CLI --> Stop
    CLI --> Delete
    CLI --> Reset
    CLI --> Recreate

    Shell -.->|"Help text & examples"| HelpOutput["CLI Help Output"]:::context
    Open -.->|"Help text & examples"| HelpOutput
    Stop -.->|"Help text & flags"| HelpOutput

    subgraph Legend
        L1["Major Edit<br/>(descriptions + examples)"]:::major-edit
        L2["Minor Edit<br/>(status messages)"]:::minor-edit  
        L3["Context/No Edit"]:::context
    end

    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#ADD8E6
    classDef context fill:#FFFFFF

Notes

• All automated tests and linting passed successfully
• Changes are purely cosmetic - no logic or API modifications
• Internal function names containing "workspace" were intentionally left unchanged
• This addresses the terminology consistency request to standardize on "instance" for user-facing text

Link to Devin run: https://app.devin.ai/sessions/589d444b5b7845078af384a16b3c99b9
Requested by: @theFong