fix: resolve ShellCheck violations in shell scripts (#91)

- SC2155 (bin/localwp-setup.sh): declare local variables separately from
  command substitution assignments to avoid masking return values (13 fixes)
- SC2034 (bin/localwp-setup.sh): remove unused PLUGIN_TEXT_DOMAIN variable
- SC2162 (bin/localwp-setup.sh): add -r flag to read to avoid backslash mangling
- SC2154 (bin/setup-test-env.sh): add shellcheck disable for PHP variables in
  heredoc that ShellCheck incorrectly identifies as unassigned shell variables
- bin/setup-test-env.sh: remove self-modifying chmod +x $0 (unnecessary and
  bad practice; file permissions should be set once in version control)
- bin/setup-test-env.sh: change == to = in POSIX [ ] test expressions
- build.sh: add ./ prefix to directory glob copies for clarity
- build.sh: use subshell (cd build || exit 1; zip ...) instead of bare cd/cd..
  to avoid SC2103 and ensure working directory is always restored

Fixes part of #20 (shell script quality issues)

build.sh

@@ -46,9 +46,9 @@ cp -R README.md LICENSE CHANGELOG.md readme.txt composer.json "$BUILD_DIR/"
 # Copy directories
 echo "Copying directories..."
 mkdir -p "$BUILD_DIR/admin" "$BUILD_DIR/includes" "$BUILD_DIR/languages" "$BUILD_DIR/assets"
-cp -R admin/* "$BUILD_DIR/admin/"
-cp -R includes/* "$BUILD_DIR/includes/"
-cp -R languages/* "$BUILD_DIR/languages/"
+cp -R ./admin/* "$BUILD_DIR/admin/"
+cp -R ./includes/* "$BUILD_DIR/includes/"
+cp -R ./languages/* "$BUILD_DIR/languages/"
 
 # Create assets directory structure
 mkdir -p "$BUILD_DIR/assets/banner" "$BUILD_DIR/assets/icon" "$BUILD_DIR/assets/screenshots"
@@ -73,9 +73,10 @@ fi
 
 # Create ZIP file.
 echo "Creating ZIP file..."
-cd build || exit 1
-zip -r "../$ZIP_FILE" "$PLUGIN_SLUG" -x "*.DS_Store" -x "*.git*" -x "*.github*"
-cd ..
+(
+	cd build || exit 1
+	zip -r "../$ZIP_FILE" "$PLUGIN_SLUG" -x "*.DS_Store" -x "*.git*" -x "*.github*"
+)
 
 # Check if ZIP file was created successfully
 if [ -f "$ZIP_FILE" ]; then