docs: state that the proxy has no native control-plane client yet

[raffaelschneider]

Fixes #7.

@scheibling was right, and the confusion was our fault. docs/PROXY-REGISTRATION.md section 2 told users to set control_plane_url, node_id and node_key "in the proxy's configuration" — those settings do not exist. I checked zentinelproxy/zentinel on main: there is no control-plane client in crates/ at all (the only grep hit for node_key is vnode_key in the consistent-hash upstream code). A stock proxy binary will never register, heartbeat, or pull a bundle.

What is actually true: the control-plane side of the workflow is implemented and exercised end to end, but the node half is driven by the built-in node simulator, not by the proxy. Connecting real proxies today needs an external sidecar speaking the node API.

This PR makes that explicit rather than leaving people to discover it:

• A status callout at the top of PROXY-REGISTRATION.md.
• Section 2 rewritten from "Configure the Proxy" to "Connect the Proxy (external sidecar required today)", listing what a minimal sidecar must do — heartbeat, poll bundles/latest, verify checksum, reload, report active_bundle_id — with a working curl heartbeat example.
• The README ## Status section now notes that "works end-to-end" means on the control-plane side, since that phrasing is where the misunderstanding starts.

No functional changes. Native proxy-side support is still worth tracking separately.

[raffaelschneider]

Merging despite red CI: this PR touches only README.md and docs/PROXY-REGISTRATION.md, and both failures are pre-existing on main (CI has been failing since 2026-03-21) and unrelated to markdown:

• Code Quality — mix format --check flags test/zentinel_cp/rollouts_test.exs formatting drift.
• Unit Tests — test/zentinel_cp/portal_test.exs:66 fails with a DBConnection.OwnershipError from the PromEx Oban poller checking out a connection outside the sandbox owner process.

Both are worth fixing separately — the PromEx poller likely needs to be disabled in the test env or given :shared sandbox mode.