docs(release-notes): retitle unreleased v6.4.0 notes as v6.4.1

v6.4.0 was prepared but never shipped (release PR #2331 closed
unmerged, no tag). The next release is v6.4.1. Rename the section and
open it with a note saying v6.4.0 never shipped and that v6.4.1 holds
back the proving-it-works-with-a-movie skill for cleanup and robustness
work before it returns. The note replaces the revert paragraph added in
#2335. Version bumps are left for the release step.

Claude-Session: https://claude.ai/code/session_01PRUnVUm4g4EcP9eNjAiT2B

RELEASE-NOTES.md

@@ -1,10 +1,10 @@
 # Superpowers Release Notes
 
-## v6.4.0 (2026-09-18)
+## v6.4.1 (2026-09-18)
 
-The new `diagnosing-superpowers` skill figures out what went wrong in a session. `executing-plans` is rebuilt as Native execution, a cheaper alternative to subagent-driven development. This release also adds support for three new harnesses: OpenCode 2.0, Muse, and Qwen Code.
+v6.4.0 was never shipped. v6.4.1 is the first release with these changes. It holds back the new `proving-it-works-with-a-movie` skill, which is getting cleanup and robustness work and will return in a later release.
 
-The `proving-it-works-with-a-movie` import (#2214) has been reverted because its code quality does not meet the release bar; the movie skill is not included.
+The new `diagnosing-superpowers` skill figures out what went wrong in a session. `executing-plans` is rebuilt as Native execution, a cheaper alternative to subagent-driven development. This release also adds support for three new harnesses: OpenCode 2.0, Muse, and Qwen Code.
 
 ### New Skills