[FEATURE] Cowork: Allow setting a default workspace folder for all sessions

Feature Request

Problem:
Every new Cowork session requires manually selecting a workspace folder via "Work in a Folder". For users who always work in the same directory, this adds friction — especially when the session depends on files in that folder (SSH keys, MCP server configs, scripts, etc.).

If you forget to select the folder, nothing works: no SSH, no custom MCP tools, no access to project files. There's no reminder or fallback.

Proposed solution:
Add a setting (in Claude Desktop preferences or claude_desktop_config.json) to define a default workspace folder that gets automatically mounted when starting a new Cowork session. Something like:

{
  "cowork": {
    "defaultWorkspaceFolder": "C:\\Users\\guill\\Documents\\WWW"
  }
}

The user could still change the folder per-session if needed, but the default would be pre-selected.

Why it matters:

• Solo developers who always work in the same project tree shouldn't have to re-select it every time
• Session-dependent tooling (SSH configs, MCP servers referencing workspace paths) breaks silently without the folder
• localAgentModeTrustedFolders already exists in the config — extending this concept to a default mount is a natural evolution

Current workaround:
Document the manual step prominently so you don't forget. That's it.

Environment:

• Windows 11
• Claude Desktop (latest, March 2026)
• Cowork mode