feat: scaffold go backend foundation

.planning/phases/001-go-backend-foundation-and-workflow-boundary/001-VERIFICATION.md

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+# Phase 1 Verification
+
+## Verdict
+
+PASS
+
+## Requirement Coverage
+
+- BACK-01: PASS. Go module and backend entrypoint exist.
+- BACK-02: PASS. `internal/workflows` exposes typed contracts and runner
+  interface.
+- BACK-03: PASS. HTTP handler receives a workflow runner dependency and does
+  not shell out.
+- BACK-04: PASS. Config includes workflow runtime, model key, and third-one
+  binary path. Default model key is `deepseek-v4-flash`.
+- BACK-05: PASS. Go source files are all under 600 lines.
+
+## Evidence
+
+- `go test ./...` passed.
+- `openspec validate bootstrap-job-tutor-platform --strict` passed.
+- Go line-count check passed.
+
+## Residual Risk
+
+The workflow runner is intentionally a stub. Real internalized `agent-farm-go`
+execution belongs to a later phase when diagnostic and grading behavior are
+implemented.