Update help documentation to use consistent 'instance' terminology

Summary

This PR updates help documentation and examples throughout the brev CLI to use consistent terminology: changing "workspace" references to "instance" and "my-app" examples to "my-instance" in user-facing help text, command descriptions, and documentation files.

Changes made:

• Updated pkg/cmd/open/open.go: Changed help strings from "workspace" to "instance" and examples from "my-app" to "my-instance"
• Updated pkg/cmd/shell/shell.go: Changed help strings from "workspace" to "instance" and examples from "my-app" to "my-instance"
• Updated documentation files (pkg/cmd/create/doc.md, pkg/cmd/start/doc.md, pkg/cmd/status/doc.md): Updated examples to use consistent "instance" terminology

Impact: 5 files changed with 89 insertions and 89 deletions (clean text replacements). This is primarily a documentation update and should not affect CLI functionality.

Review & Testing Checklist for Human

• Test help text display: Run brev open --help and brev shell --help to verify the updated help text displays correctly with "instance" terminology
• Verify functionality: Test that brev open and brev shell commands still work as expected (functionality should be unaffected)
• Check completeness: Search the codebase for any remaining user-facing "workspace" or "my-app" references that might need updating
• Validate examples: Ensure the new "my-instance" examples in help text are appropriate and follow current naming conventions

Test plan: Try running basic CLI help commands and a few actual commands to ensure the terminology updates are visible to users and don't break existing functionality.

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    OpenCmd["pkg/cmd/open/open.go<br/>openLong, openExample, Short"]:::major-edit
    ShellCmd["pkg/cmd/shell/shell.go<br/>openLong, openExample"]:::major-edit
    CreateDoc["pkg/cmd/create/doc.md<br/>Help examples"]:::minor-edit
    StartDoc["pkg/cmd/start/doc.md<br/>Help examples"]:::minor-edit  
    StatusDoc["pkg/cmd/status/doc.md<br/>Help examples"]:::minor-edit
    
    CLI["brev CLI<br/>User Interface"]:::context
    
    CLI --> OpenCmd
    CLI --> ShellCmd
    OpenCmd -.-> CreateDoc
    ShellCmd -.-> StartDoc
    StatusDoc -.-> CLI
    
    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:#ADD8E6
    classDef context fill:#FFFFFF

Notes

• This PR addresses inconsistent terminology where "workspace" and "my-app" were used in help documentation
• The main CLI description in pkg/cmd/cmd.go already correctly uses "brev client for managing instances" so no changes were needed there
• Internal function names and variable names that use "workspace" were left unchanged as they are not user-facing
• Unable to test locally due to build environment issues, so human testing of the actual CLI help output is important

Session details:

• Link to Devin run: https://app.devin.ai/sessions/d767e8c9fa3e4eacacdbba260309eb75
• Requested by: @theFong