squad doctor falsely reports decisions.md missing for two-layer state backend

[serbrech]

Symptom

After initializing Squad with stateBackend: "two-layer", squad doctor reports decisions.md missing from the working tree.

Expected behavior

For two-layer/orphan state backends, decisions.md is intentionally stored on the squad-state branch, so doctor should validate it through the configured state backend instead of requiring .squad/decisions.md in the working tree.

Actual behavior

Doctor checks for .squad/decisions.md as a filesystem file and fails even when decisions.md exists in squad-state.

Notes

This also affects the legacy stateBackend: "git-notes" alias, which the SDK normalizes to two-layer.