fix(ci): run rc releases from next branch

docs/release.md

@@ -12,6 +12,11 @@ KTX has two npm release channels:
 - `rc` publishes prereleases such as `0.1.0-rc.2` to the npm `next` tag.
 - `stable` publishes normal releases such as `0.1.0` to the npm `latest` tag.
 
+Run rc releases from the source branch you want to publish. The workflow
+creates or updates the `next` prerelease branch from that source branch before
+running semantic-release, because semantic-release requires a dedicated
+prerelease branch in addition to the stable `main` branch.
+
 Run stable releases only from `main`. The workflow rejects stable releases from
 other branches.
 
@@ -43,8 +48,9 @@ publishing to npm.
    if semantic-release doesn't find a releasable commit.
 7. Run the workflow.
 
-The dry-run uses the same semantic-release configuration as a live release. It
-doesn't publish to npm and doesn't commit release files.
+The dry-run uses the same semantic-release configuration as a live release. For
+rc releases, it can create or update the `next` branch. It doesn't publish to
+npm and doesn't commit release files.
 
 ## Publish an rc release
 
@@ -53,15 +59,16 @@ promoting to `latest`.
 
 1. Open **Actions** in GitHub.
 2. Select **KTX Release**.
-3. Select the branch to release from.
+3. Select the source branch to release from.
 4. Set **release_kind** to `rc`.
 5. Set **publish_live** to `true`.
 6. Optional: Set **force_release** to `true`.
 7. Run the workflow.
 
-The workflow publishes `@kaelio/ktx` with `--access public --tag next`, runs the
-published package smoke test, creates a GitHub release, and commits
-`CHANGELOG.md`, `package.json`, and `release-policy.json`.
+The workflow merges the selected source branch into `next`, publishes
+`@kaelio/ktx` with `--access public --tag next`, runs the published package
+smoke test, creates a GitHub release, and commits `CHANGELOG.md`,
+`package.json`, and `release-policy.json` on `next`.
 
 ## Publish a stable release