feat: add diagnostic interview loop

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

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+# Phase 2: Diagnostic Interview Loop - Context
+
+**Gathered:** 2026-04-26
+**Status:** Ready for planning
+**Source:** GSD continuation after Phase 1 completion
+
+<domain>
+## Phase Boundary
+
+Phase 2 proves the first job-seeker loop from target role/stack selection
+through a graded diagnostic answer. It should create a thin backend product
+surface for diagnostic sessions while avoiding persistent storage and real LLM
+calls until later phases require them.
+
+</domain>
+
+<decisions>
+## Implementation Decisions
+
+### Persistence
+
+- Use an in-memory session store in Phase 2.
+- Do not add a database or migrations yet.
+- Persisting original answers and grading evidence means preserving them inside
+  the in-memory diagnostic session record for this phase.
+
+### Interview Track
+
+- Use the Backend Developer Interview track from
+  `docs/planning/INTERVIEW_TRACKS.md`.
+- Seed questions should cover the canonical concept clusters, starting with a
+  small representative set.
+
+### Workflow Boundary
+
+- Grading must go through the typed `workflows.Runner` interface.
+- The default implementation can remain deterministic/stubbed, but it must
+  return typed `GradedAnswer` data rather than freeform prose.
+- HTTP handlers must not shell out or parse arbitrary assistant text.
+
+</decisions>
+
+<canonical_refs>
+## Canonical References
+
+Downstream agents MUST read these before planning or implementing.
+
+### Product and Track
+
+- `docs/planning/PRD.md` - diagnostic interview product flow.
+- `docs/planning/INTERVIEW_TRACKS.md` - first track and concept seed list.
+- `docs/planning/WORKFLOW_CONTRACTS.md` - typed workflow result shape.
+
+### Engineering and Requirements
+
+- `docs/planning/ENGINEERING.md` - 600-line, SOLID, KISS, YAGNI constraints.
+- `.planning/REQUIREMENTS.md` - INT-01 through INT-06 requirements.
+- `.planning/ROADMAP.md` - Phase 2 goal and success criteria.
+- `openspec/changes/bootstrap-job-tutor-platform/specs/job-seeker-tutor/spec.md`
+  - diagnostic and first-track requirements.
+- `openspec/changes/bootstrap-job-tutor-platform/specs/tutor-workflows/spec.md`
+  - typed workflow requirements.
+
+</canonical_refs>
+
+<specifics>
+## Specific Ideas
+
+- Add `internal/interview` for sessions, questions, answers, and in-memory store.
+- Add endpoints:
+  - `POST /api/v1/diagnostic-sessions`
+  - `GET /api/v1/diagnostic-sessions/{id}`
+  - `POST /api/v1/diagnostic-sessions/{id}/answers`
+- Keep routing simple with standard library `http.ServeMux`.
+- Add deterministic grading in the workflow stub so tests can prove typed
+  evidence is recorded.
+
+</specifics>
+
+<deferred>
+## Deferred Ideas
+
+- Real `third-one` grading execution.
+- Database persistence.
+- Authentication and user identity provider integration.
+- Memory extraction and durable learner memory.
+- Frontend UI.
+
+</deferred>
+
+---
+*Phase: 002-diagnostic-interview-loop*
+*Context gathered: 2026-04-26*