Run the Go SDK e2e suite from the orchestrator and gate its coverage like the TS SDK #518

The TypeScript SDK has three coverage gates in .testcoverage.yml — ts-unit, ts-e2e, ts-total — because it has two coverage-producing suites: vitest unit tests, and the e2e suite the orchestrator (scripts/orchestrator) drives against a live cover binary. The Go SDK has only go-sdk, because it has only one: make test-sdk-go.

The Go SDK does have an e2e suite (clients/go/e2e_test.go, //go:build e2e), but it runs nowhere automatic — make test-sdk-go-e2e needs a server someone else started (WAVEHOUSE_URL / WAVEHOUSE_AUTH), collects no coverage, and CI never invokes it. So the asymmetry Eric flagged in #434 is real: the Go SDK's e2e half is not held to the same standard as the TypeScript SDK's.

What to do

1. Run the Go SDK e2e suite from scripts/orchestrator after the vitest harness, against the same ClickHouse testcontainer + bin/wavehouse-cov it already stands up — forwarding WAVEHOUSE_URL (and an auth token if the e2e server config grows one). make test-e2e then covers both SDKs' e2e halves, matching what "E2E" already means in this repo.
2. Collect covdata from that run (-coverpkg=./..., GOCOVERDIR → tmp/coverage/go-sdk-e2e/data).
3. Generalize scripts/cov's merge so the nested module gets its own merged total, and split the gate into go-sdk-unit / go-sdk-e2e / go-sdk-total — mirroring ts-unit / ts-e2e / ts-total.
4. Drop make test-sdk-go-e2e (or keep it as the local, no-orchestrator escape hatch) and update docs/src/content/docs/development.md + .claude/commands/cover.md.

Why it is not in #434

It is a new suite plus new coverage-merge machinery in scripts/cov, not a review fix — and it puts a new suite on CI's long-pole e2e job, which wants its own validation pass. Tracked here per AGENTS.md §Review Response.

Surfaced by @EricAndrechek reviewing #434 (.testcoverage.yml thread).