chore: add e2e initial tests that check on cluster creation flow

[Davidonium]

Bootstraps an end-to-end test suite that exercises the real qcloud binary against Qdrant Cloud, plus the plumbing to run it on a schedule.

• A reusable harness that resolves the qcloud binary once per run (downloaded and sha256-verified against the release checksums, cached across runs), and gives tests an isolated CLI environment with structured (--json) output helpers.
• Domain helpers for common flows (cluster create / wait / delete) with automatic cleanup, plus a best-effort sweep for orphaned e2e resources.
• First smoke test covering the cluster create → wait → key create → delete path.
• A nightly GitHub Actions workflow and a make e2e target.
• A short README documenting required env vars, how to target a locally-built binary, and safety expectations.

Opt-in via QCLOUD_E2E=1 — the default go test ./... stays network-free.

Test plan

• go test ./test/e2e/... skips without QCLOUD_E2E set.
• make e2e succeeds end-to-end against a staging account. (Did this locally)
• Workflow run via workflow_dispatch completes green.
• Workflow run via cron completes green.

Blocking

What Account ID and what environment should I use for the tests? I need to set up the secrets properly.