Point rust-publish-release workflow header to RELEASING.md

The 23-line setup checklist duplicated content already in
rust/RELEASING.md. One-line pointer is enough.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tclem

.github/workflows/rust-publish-release.yml

@@ -1,27 +1,8 @@
 name: "Rust SDK: Publish Release"
 
 # Publishes the `copilot-sdk` crate to crates.io when a release-plz
-# version-bump PR is merged to `main`. release-plz detects that the
-# current HEAD's `Cargo.toml` version is newer than the latest
-# `rust-vX.Y.Z` tag, then runs `cargo publish` and creates the tag +
-# GitHub Release.
-#
-# Does NOTHING on non-release commits — safe to run on every push.
-#
-# ## Setup checklist (one-time, before enabling this workflow)
-#
-# 1. Reserve the `copilot-sdk` crate name on crates.io via the owner
-#    account (the GitHub service account or a human maintainer with
-#    crates.io 2FA).
-# 2. Generate a crates.io API token scoped to publish `copilot-sdk`
-#    only (Account Settings → API Tokens → "New Token" → restrict
-#    crates to `copilot-sdk`).
-# 3. Add it to this repository's secrets as `CARGO_REGISTRY_TOKEN`.
-# 4. Rotate the token annually and whenever maintainers change.
-#
-# Until the setup checklist is complete, this workflow will fail on
-# `cargo publish`. That's intentional — it keeps accidental publishes
-# from happening before the repo is ready.
+# version-bump PR is merged to `main`. See rust/RELEASING.md for the
+# full release process and one-time setup (CARGO_REGISTRY_TOKEN, etc).
 
 on:
   push: