De-fork e2e: trace-golden harness (session trace as behavioral snapshot oracle)

Novel e2e technique: drive a real headless session with a scripted model, capture the emitted trace, normalize (allowlist projection), and partial-order-match against a committed golden. Codex-scoped to S5 observable native/plugin parity + S7 continuation invariants (NOT an S3 security oracle — tracing is failure-suppressing). Files: packages/opencode/test/altimate/trace-golden/{normalize,match,driver}.ts + scenarios. Technique doc: docs/internal/2026-07-18-trace-golden-e2e-technique.md. Test/infra only, zero product changes.