feat(boulder-state): add session_origins tracking for direct vs appended sessions

- Add session_origins field to BoulderState type
- Track direct vs appended session origins in storage layer
- Add migration logic for existing state files
- Add comprehensive tests for session origin tracking

🤖 Generated with assistance of OhMyOpenCode

src/features/boulder-state/storage.test.ts

@@ -151,7 +151,6 @@ describe("boulder-state", () => {
       // then
       expect(result?.session_origins).toEqual({})
     })
-
     test("should read valid boulder state", () => {
       // given - valid boulder.json
       const state: BoulderState = {