♻️ Move script-header rationale into development/

branch.sh, finish.sh, release.sh, runner-image.sh and release-notes.sh
carried multi-paragraph 'how we got here / rejected' essays in comments. They
now live in the matching development/ category file, and each script keeps a
one-line pointer so the rationale is single-homed and cannot drift.

scripts/runner-image.sh

@@ -8,6 +8,9 @@ set -euo pipefail
 # pulls the image by name.
 #
 # Usage: scripts/runner-image.sh [--push]
+#
+# Why the image is baked, its two invariants, and the coordinated Node-bump
+# steps: development/ci.md.
 
 IMAGE_REPO="gitea.e1nsnull.de/tmu/act-ci"