test(cli): address review — assert schema-show success, document exit-code stance, add e2e opt-out

- The drift-heal verification now asserts `schema show` succeeded and
  produced a schema before checking the rogue field's absence (a failed
  command previously made the negative assertion vacuously pass).
- cluster_cli documents why it deliberately does not assert exit codes
  (blocked applies exit non-zero by contract while emitting the structured
  output callers assert on).
- The comprehensive lifecycle e2es honor OMNIGRAPH_SKIP_SYSTEM_E2E=1
  (graceful skip-with-message, the S3-gate pattern) for constrained
  sandboxes; requirements + suppression documented in testing.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/dev/testing.md

@@ -69,6 +69,10 @@ CI runs three S3-backed tests against a containerized RustFS server (`.github/wo
 
 Locally, set `OMNIGRAPH_S3_TEST_BUCKET` (and the usual `AWS_*` vars including `AWS_ENDPOINT_URL_S3` for non-AWS) before running. Without those, S3 tests skip gracefully.
 
+## System e2e requirements and suppression
+
+The CLI system tests (`system_local.rs`) spawn the workspace-built `omnigraph` and `omnigraph-server` binaries (cargo provides paths via `CARGO_BIN_EXE_*`), bind ephemeral localhost ports, and use local-FS temp dirs — no external services, no env vars required; they run in the default `cargo test --workspace`. The comprehensive cluster lifecycle e2es (multi-server-restart flows) honor an opt-out for constrained sandboxes: set `OMNIGRAPH_SKIP_SYSTEM_E2E=1` to skip them with a logged message (the same graceful-skip pattern as the S3 gate). Cargo-native filtering also works: `cargo test --test system_local -- --skip local_cluster`.
+
 ## OpenAPI drift
 
 `crates/omnigraph-server/tests/openapi.rs` regenerates `openapi.json` and diffs against the checked-in copy. CI auto-commits the regeneration on same-repository PRs and otherwise runs in strict-check mode (env: `OMNIGRAPH_UPDATE_OPENAPI`).