♻️ 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/release-notes.sh

@@ -9,6 +9,8 @@ set -eu
 # `v1.2.3` and `1.2.3` match the `## [1.2.3]` heading. Prints to stdout and
 # exits non-zero when the tag has no section, so a release can never publish
 # with an empty body.
+#
+# Why: development/publishing.md § Release notes are extracted from the changelog.
 
 TAG="${1:-}"
 CHANGELOG="${CHANGELOG:-CHANGELOG.md}"