feat: add diagnostic interview loop

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

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+# Phase 2 Verification
+
+## Verdict
+
+PASS
+
+## Requirement Coverage
+
+- INT-01: PASS. Diagnostic session request accepts target role, stack, and
+  interview timeline.
+- INT-02: PASS. Diagnostic sessions can progress to `complete` after all seed
+  questions are answered.
+- INT-03: PASS. Backend Developer Interview questions are generated from the
+  role-specific seed catalog.
+- INT-04: PASS. Answers are graded through the typed workflow runner boundary.
+- INT-05: PASS. Weak or partial answers receive typed follow-up recommendations.
+- INT-06: PASS. Original answer text and grading evidence are preserved in the
+  in-memory session record.
+
+## Evidence
+
+- `go test ./...` passed.
+- `openspec validate bootstrap-job-tutor-platform --strict` passed.
+- Live `GET /healthz` smoke passed.
+- Live diagnostic create/answer/get smoke passed.
+- Go source line-count check passed.
+
+## Residual Risk
+
+Persistence is intentionally in-memory. Data is lost on process restart. Phase 3
+should decide whether learner memory remains in-memory for proof or introduces a
+small persistent boundary.