Two fixes for the model configuration bleeding between simultaneous
GSD instances that share the same global settings.json.
## Root Cause
1. `setDefaultModelAndProvider()` always persisted to `~/.gsd/agent/settings.json`
(global), so when either instance's interactive mode changed models (via
Ctrl+P or /model), it overwrote the other instance's saved default.
2. When auto-mode dispatched a new unit (after context wipe), if no
per-unit-type model preference was configured, the session picked up
the default from the now-contaminated global settings file.
## Fix 1: Project-scoped model persistence (settings-manager.ts)
`setDefaultModelAndProvider()`, `setDefaultModel()`, and `setDefaultProvider()`
now persist to project-level settings (`.pi/settings.json`) when a project
settings file exists, falling back to global only when no project context
is available. This prevents concurrent instances from overwriting each
other's model choice.
Added `hasProjectSettingsFile()` helper to detect project context.
## Fix 2: Auto-mode model capture (auto.ts)
Captures the session's model at auto-mode start (`autoModeStartModel`).
At each unit dispatch, if no model preference is configured for the unit
type, the captured model is re-applied with `persist: false`. This
ensures each auto-mode session maintains its own model regardless of
what other instances write to the shared settings file.
## Tests
3 new tests covering:
- Project settings file isolates model from global
- Two projects have independent model configs
- autoModeStartModel concept prevents model drift
All 448 existing tests pass.
Fixes#650
cdf42fe001