fix(ci): count a modified changeset, and grow this change's release note

One change gets one changeset. When a stack lands forward rather than as a
single merge-down, every PR after the first finds that changeset already on
`main` from the PR below it — so its own diff adds none and the check fails,
even though the release note exists and covers it. Counting additions only also
made the "grow the changeset as the stack lands" rule in CLAUDE.md impossible to
satisfy, since growing it is a modification.

The check now counts added OR modified changesets, and this PR extends the
existing note with the behaviour it actually adds rather than filing a second
entry for something that ships once.

Author: theCodeDrift

.changeset/engine-partitioned-layout.md

@@ -6,4 +6,6 @@ Partition `.taskless/` by rule engine. Migration `0004` moves ast-grep rules to
 
 The directory a rule sits in now **is** its engine: dispatch reads the path and never parses a rule file to decide who owns it. `check` runs ast-grep against the committed `.taskless/sg/sgconfig.yml` instead of generating an ephemeral config each run.
 
+A rule engine the CLI does not recognize is now rejected with a message instead of failing silently: an unsupported engine from the server previously exited 0 with no output, which read as success.
+
 Existing projects keep working without action. The pre-`0004` `.taskless/rules/` still runs as ast-grep, and a delivered rule that names no engine is still treated as ast-grep — a rule engine this CLI does not recognize is rejected rather than guessed at. A migration that would have to merge a file into an engine directory now refuses up front with `SCAFFOLD_CONFLICT` rather than failing part-way.

.github/workflows/require-changeset.yml

@@ -78,9 +78,16 @@ jobs:
               ;;
           esac
 
-          # A changeset is any new `.changeset/*.md` added by this PR, other than
-          # the template README.
-          ADDED=$(git diff --name-only --diff-filter=A "$BASE_SHA...$HEAD_SHA" -- '.changeset/*.md' \
+          # A changeset is any `.changeset/*.md` this PR adds OR modifies, other
+          # than the template README.
+          #
+          # Modifications count because one change gets ONE changeset: when a
+          # stack lands forward, the second PR onward finds the release note
+          # already on `main` from the PR below it, and the right move is to
+          # extend that file rather than add a second entry for something that
+          # ships once. Counting additions only made "grow the changeset as the
+          # stack lands" impossible to satisfy.
+          ADDED=$(git diff --name-only --diff-filter=AM "$BASE_SHA...$HEAD_SHA" -- '.changeset/*.md' \
             | grep -viE '/README\.md$' || true)
 
           if [ -z "$ADDED" ]; then