Add a setting

AppPreferences in src-tauri/src/preferences/model.rs is the wire struct. Import src/types/generated/AppPreferences.ts. Do not declare a parallel TypeScript interface.

1. Add the field on AppPreferences with a serde default so an existing preferences file still loads. A JSON shape change goes through src-tauri/src/preferences/migrations.rs. Load and save stay in src-tauri/src/preferences/storage.rs, off the async worker pool.
2. A value that changes how an agent spawns is resolved in src-tauri/src/preferences/resolver/cascade.rs (explicit, then mesh, then application). The spawn path and get_resolved_harness_view both use that cascade. Do not re-derive the order in a command or a React component.
3. A per-harness launch default (model, effort, extra arguments, permission mode) belongs in src-tauri/src/preferences/launch_configurations.rs, not a one-off field on the modal.
4. Put the control in the existing pane under src/components/AppSettings: GeneralPane.tsx, ProvidersPane.tsx, HarnessesPane.tsx, AutopilotPane.tsx (Circuit limits), PromptsPane.tsx (spawn prompts) or RemoteAccessPane.tsx, all in src/components/AppSettings. Colours come from src/App.css. Do not hardcode a colour.
5. Document the default, what it changes, and how to undo it in docs/user-guide.md.

When the wire struct changes, run cargo test from src-tauri/ and commit src/types/generated.

From src-tauri/: cargo test --lib preferences. For a visible control, also run the unit test that renders the pane you edited (npx vitest run <file> --pool=threads from the worktree root, with NODE_ENV=test).