feat(ci): reach a milestone's release notes back over its patches

A promotion tags a commit that is already tagged, so GoReleaser's own
previous tag makes the notes on a release consolidating six patches
describe one merge. Emits the last release at the level being cut instead.

Also drops the named-version path: the manual pipeline no longer offers one.

Claude-Session: https://claude.ai/code/session_01ShuAy8q8ZfPi8KHxwc8JpQ

.github/scripts/next-version.sh

@@ -4,66 +4,74 @@
 # nothing in the tree holds it: no commit has to land on master to advance a
 # version, and a release cannot disagree with a package.json someone edited.
 #
-# BUMP is major, minor or patch. VERSION overrides it with a version named
-# outright. Writes `version`, `tag` and `released_tag` to $GITHUB_OUTPUT when it
-# is set; `released_tag` is the release this one follows, and is empty in a
-# repository that has never cut one.
+# BUMP is major, minor or patch. Writes `version`, `tag`, `released_tag` and
+# `previous_tag` to $GITHUB_OUTPUT when it is set. `released_tag` is the release
+# this one follows, and is the commit a promotion re-cuts. `previous_tag` is how
+# far back the release notes should reach.
 set -euo pipefail
 
 bump="${BUMP:-patch}"
-named="${VERSION:-}"
-
-semver='^[0-9]+\.[0-9]+\.[0-9]+(-[0-9A-Za-z]+(\.[0-9A-Za-z]+)*)?$'
 
 # Only a stable tag counts as a release. v0.0.1-rc4 is a candidate for 0.0.1, so
 # counting a patch off it would skip the very version it was a candidate for.
 # `sort -V` puts 0.10.0 above 0.9.0, which a lexical sort does not, and
 # `sed -n p` reports no matches as an empty line rather than as the failure
 # `grep` would return under pipefail.
-released="$(git tag -l 'v*' \
-  | sed -n 's/^v\([0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\)$/\1/p' \
-  | sort -V \
-  | tail -1)"
-released_tag=""
-[ -n "$released" ] && released_tag="v$released"
+releases() { # <sed script selecting the tags to consider>
+  git tag -l 'v*' | sed -n "$1" | sort -V
+}
 
-if [ -n "$named" ]; then
-  if [[ ! "$named" =~ $semver ]]; then
-    echo "next-version: '$named' is not a version this releases" >&2
-    echo "next-version: a version is MAJOR.MINOR.PATCH with an optional -prerelease, e.g. 0.1.0 or 1.0.0-rc1" >&2
+stable='s/^v\([0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\)$/\1/p'
+released="$(releases "$stable" | tail -1)"
+released_tag=""
+if [ -n "$released" ]; then released_tag="v$released"; fi
+
+base="${released:-0.0.0}"
+IFS=. read -r major minor patch <<<"$base"
+
+case "$bump" in
+  major)
+    version="$((major + 1)).0.0"
+    level='s/^v\([0-9][0-9]*\.0\.0\)$/\1/p'
+    ;;
+  minor)
+    version="$major.$((minor + 1)).0"
+    level='s/^v\([0-9][0-9]*\.[0-9][0-9]*\.0\)$/\1/p'
+    ;;
+  patch)
+    version="$major.$minor.$((patch + 1))"
+    level=""
+    ;;
+  *)
+    echo "next-version: '$bump' is not a bump; use major, minor or patch" >&2
     exit 1
-  fi
-  version="$named"
-  from="named outright"
-else
-  base="${released:-0.0.0}"
-  from="a $bump off ${base}"
-  IFS=. read -r major minor patch <<<"$base"
-  case "$bump" in
-    major) version="$((major + 1)).0.0" ;;
-    minor) version="$major.$((minor + 1)).0" ;;
-    patch) version="$major.$minor.$((patch + 1))" ;;
-    *)
-      echo "next-version: '$bump' is not a bump; use major, minor or patch" >&2
-      exit 1
-      ;;
-  esac
+    ;;
+esac
+
+# A patch follows the release before it, which is what GoReleaser assumes on its
+# own, so it is left alone to assume it. A milestone consolidates every patch
+# since the last release at its own level, and its notes have to reach back that
+# far or they describe the one merge that happened to be last. With nothing at
+# that level yet, they reach back to the first release there has ever been.
+previous_tag=""
+if [ -n "$level" ]; then
+  previous="$(releases "$level" | tail -1)"
+  previous="${previous:-$(releases "$stable" | head -1)}"
+  if [ -n "$previous" ]; then previous_tag="v$previous"; fi
 fi
 
 tag="v$version"
 
-# A tag that is already there means this version was already cut. Moving it
-# would relabel a release that people have installed.
-if git rev-parse -q --verify "refs/tags/$tag" >/dev/null; then
-  echo "next-version: $tag is already tagged, so there is nothing to release at $version" >&2
-  exit 1
+echo "next-version: releasing $version, a $bump off $base"
+if [ -n "$previous_tag" ]; then
+  echo "next-version: the notes reach back to $previous_tag"
 fi
 
-echo "next-version: releasing $version, $from"
 if [ -n "${GITHUB_OUTPUT:-}" ]; then
   {
     echo "version=$version"
     echo "tag=$tag"
     echo "released_tag=$released_tag"
+    echo "previous_tag=$previous_tag"
   } >> "$GITHUB_OUTPUT"
 fi