Add workspaceMode as a first-class ProjectSettings boolean that controls
whether the project root is treated as a workspace parent (multi-repo)
or a single git repo.
- ProjectSettings type + DEFAULT_PROJECT_SETTINGS: workspaceMode?: boolean
- CLI registerProjectInteractive: when sub-repos are detected, ask the
user to confirm workspace mode instead of auto-applying
- TaskStore.updateSettings: when workspaceMode is toggled on, detect
sub-repos and persist workspace.json; when toggled off, remove it
- Dashboard SettingsModal GeneralSection: workspace mode toggle checkbox
This lets users change workspace mode per-project at any time via the
dashboard Settings or PUT /settings API.
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