Add Tempo-gated e2e test for OTel tracing

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

docs/plans-executions/2026-08-24-1224-tracing-e2e.md

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+# Execution log: e2e test — OTel tracing against real Tempo
+
+Plan: `docs/plans/2026-08-24-1224-tracing-e2e.md`
+
+- [x] Step 1 — `test/e2e/tracing_test.go`
+- [x] Step 2 — Makefile
+- [x] Step 3 — Docs
+
+## Steps 1–3 — spec, Makefile, docs (one commit)
+
+The three steps landed together — the spec is one new file and the other
+two are its wiring. `Describe("OTel tracing", Ordered)` is fully
+self-contained (own ns create → `make install`/`make deploy` → teardown)
+because Ginkgo randomizes top-level container order, so it cannot share the
+Manager Describe's deployment. It reuses the package-level `namespace` /
+`managerImage` and the suite's idioms (`utils.Run`, curl-pod with the
+restricted-PSS overrides JSON, log-substring `Eventually`s).
+
+Judgment calls beyond the plan's letter:
+
+- The squid pre-pull (`docker pull` + `kind load`) is **best effort** — a
+  missing docker binary logs a note and continues rather than failing the
+  spec; the 5m Ready timeout still covers an in-cluster pull.
+- The OTLP preflight pod prints per-attempt HTTP codes and a final
+  `OTLP_OK`/`OTLP_UNREACHABLE` marker; the assertion quotes the pod's
+  output, so an unreachable endpoint names itself in the failure.
+- `kubectl set env` is passed the literal
+  `OTEL_RESOURCE_ATTRIBUTES=...$(POD_NAME)...` string via `exec.Command` —
+  no shell involved, kubectl stores `$()` verbatim, and the in-place update
+  keeps the var after the downward-API vars it references.
+- Tempo helpers are stdlib-only; `/api/traces/<id>` is decoded as
+  OTLP-JSON (`batches[].scopeSpans[].spans[].name`), which is Tempo's
+  actual shape (not Jaeger's).
+
+Verified so far: `go vet -tags=e2e ./...` clean. The live run against
+Tempo (`TEMPO_URL=http://192.168.0.30:3200
+OTLP_ENDPOINT=http://192.168.0.30:4318 make test-e2e`) is recorded below
+once it has been executed.