Release notes come out empty (and org announcement crashes) when release PRs are unlabeled

[rahlk]

Summary

The release workflow publishes empty release notes and the org-announcement step crashes
with gh: Body can't be blank whenever the release's PRs aren't tagged with the category labels.

Observed on v1.4.4 (run 29935114929):
the GitHub release body was blank, the auto-created repo discussion had only the machine footer, and
the org-mirror step failed (masked by continue-on-error).

Root cause

release.yml builds the release body from mikepenz/release-changelog-builder-action
(steps.gen_changelog.outputs.changelog). Its release_config.json categorizes PRs strictly by
label (fix, bug, kind/feature, …) and the default template emits only categorized PRs.
The v1.4.4 PRs (#285, #286, #287) were unlabeled, so the log shows:

✒️ Wrote 0 categorized pull requests down

→ empty changelog output → blank release body → blank BODY for the org createDiscussion
mutation → Body can't be blank. This recurs for any release whose PRs lack the exact labels.

(Also latent: steps.changelog_reader reads version: ${{ steps.tag_name.outputs.current_version }},
but there is no tag_name step, so that value is always empty; the reader's output is unused.)

Proposed fix

Source the release body from the hand-written CHANGELOG.md section for the tag (the SDK already
keeps a Keep-a-Changelog file, and the announcement should be "grounded in CHANGELOG.md, not the
auto-grouping"), and hard-fail the release if that section is empty so we never publish/announce
a blank body again. Drop the label-dependent mikepenz step and orphaned release_config.json.