feat: add diagnostic interview loop

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

@@ -0,0 +1,77 @@
+# Phase 2 Plan: Diagnostic Interview Loop
+
+**Status:** Ready for execution
+**Phase Goal:** Prove the first job-seeker loop from role selection through
+graded diagnostic interview.
+
+## Requirements Covered
+
+- INT-01: User can select target role, stack, and interview timeline.
+- INT-02: User can complete a diagnostic technical interview.
+- INT-03: System can generate role-specific interview questions.
+- INT-04: System can grade user answers against explicit rubrics.
+- INT-05: System can ask targeted follow-up questions for weak answers.
+- INT-06: System preserves original answers and grading evidence.
+
+## Tasks
+
+### 1. Add interview domain package
+
+- Create `internal/interview`.
+- Define session, question, answer, and store types.
+- Add in-memory store implementation.
+- Add backend developer diagnostic question catalog.
+
+### 2. Add diagnostic service
+
+- Create session from target role, stack, and optional interview timeline.
+- Select first diagnostic questions from the backend developer track.
+- Record submitted answers.
+- Invoke `workflows.Runner.GradeInterviewAnswer`.
+- Attach typed grading result and evidence to the answer record.
+
+### 3. Add HTTP endpoints
+
+- `POST /api/v1/diagnostic-sessions`
+- `GET /api/v1/diagnostic-sessions/{id}`
+- `POST /api/v1/diagnostic-sessions/{id}/answers`
+
+### 4. Extend workflow stub
+
+- Return deterministic typed grades.
+- Include follow-up recommendation for weak answers.
+- Include evidence references.
+
+### 5. Add tests
+
+- Domain tests for session creation and answer grading.
+- HTTP tests for create/get/answer flow.
+- Existing config/workflow tests remain passing.
+
+### 6. Update GSD/OpenSpec state
+
+- Mark INT-01 through INT-06 complete if all success criteria pass.
+- Add Phase 2 summary and verification artifacts.
+
+## Verification
+
+```powershell
+gofmt -w cmd internal
+go test ./...
+openspec validate bootstrap-job-tutor-platform --strict
+```
+
+Run Go line-count check and confirm every manually authored Go file is at or
+below 600 lines.
+
+## Out of Scope
+
+- Database persistence.
+- Authentication.
+- Real LLM grading.
+- Durable learner memory.
+- Progression map.
+- Frontend.
+
+---
+*Plan created: 2026-04-26*