fix(ci): skip CHANGELOG script for feature/standalone #4700

Problem:
"prerelease" and "package test" CI jobs are failing:

     error: changelog data file already exists: /codebuild/output/src1532600206/src/github.com/aws/aws-toolkit-vscode/.changes/2.19.0-SNAPSHOT.json

because 3890da4 did a "fake release" on
`feature/standalone`, then later `public/master` merged-in
(1170aa9) more changelog entries (without bumping the version).

Solution:
- don't generate CHANGELOG for feature/standalone
- revert 3890da4 (because 2.19 was not actually released yet)
- TODO: move `.changes/` to subprojects

Author: justinmk3

.changes/2.19.0-SNAPSHOT.json

@@ -0,0 +1,4 @@
+{
+	"type": "Feature",
+	"description": "Amazon Q Feature Dev: Use project context from workspace root directory instead of the /src folder"
+}

.changes/next-release/Feature-b9e8098d-e1a6-46e8-b13e-137b3ac6f95e.json

@@ -1,7 +1,3 @@
-## 2.19.0-SNAPSHOT 2024-04-10
-
-- **Feature** Amazon Q Feature Dev: Use project context from workspace root directory instead of the /src folder
-
 ## 2.18.0 2024-04-04
 
 - **Bug Fix** Amazon Q Feature Dev: Fix followups after hitting iteration limit

CHANGELOG.md

@@ -26,8 +26,9 @@ phases:
                 npm version --no-git-tag-version "$VERSION" -w packages/${TARGET_EXTENSION}
                 # Call npm ci because 'createRelease' uses ts-node
                 npm ci
+            # TODO: fix createRelease (generates CHANGELOG) for "amazonq".
             - |
-                npm run createRelease
+                echo "$CODEBUILD_WEBHOOK_HEAD_REF" | >/dev/null grep master && npm run createRelease || true
             - |
                 git add packages/${TARGET_EXTENSION}/package.json
                 git add package-lock.json