feat: add diagnostic interview loop

.planning/phases/002-diagnostic-interview-loop/002-SUMMARY.md

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+# Phase 2 Summary
+
+**Status:** Complete
+**Completed:** 2026-04-26
+
+## Delivered
+
+- Added in-memory diagnostic interview domain package.
+- Added Backend Developer Interview seed question catalog.
+- Added diagnostic session create/get/answer service.
+- Added session status that becomes `complete` after all diagnostic questions
+  have answers.
+- Added HTTP endpoints:
+  - `POST /api/v1/diagnostic-sessions`
+  - `GET /api/v1/diagnostic-sessions/{id}`
+  - `POST /api/v1/diagnostic-sessions/{id}/answers`
+- Extended workflow stub to return deterministic typed grading results.
+- Added grading evidence to `GradedAnswer`.
+- Added domain, workflow, and HTTP flow tests.
+
+## Files Added
+
+- `internal/httpapi/diagnostic.go`
+- `internal/httpapi/diagnostic_test.go`
+- `internal/interview/catalog.go`
+- `internal/interview/service.go`
+- `internal/interview/service_test.go`
+- `internal/interview/store.go`
+- `internal/interview/types.go`
+
+## Verification
+
+```powershell
+gofmt -w cmd internal
+go test ./...
+openspec validate bootstrap-job-tutor-platform --strict
+```
+
+Additional live smoke checks:
+
+- `GET /healthz` returned status `ok`.
+- Diagnostic create/answer/get flow returned a session id, 3 questions, a
+  `solid` typed grade, 1 evidence item, and 1 stored answer.
+
+## Deferred
+
+- Durable database persistence.
+- Authentication.
+- Real `third-one` grading calls.
+- Learner memory extraction and readiness progression.