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)

bin/setup-test-env.sh

@@ -1,8 +1,5 @@
 #!/bin/bash
 
-# Make this script executable
-chmod +x "$0"
-
 # Check if environment type is provided
 if [ -z "$1" ]; then
 	echo "Usage: $0 [single|multisite|playground-single|playground-multisite]"
@@ -49,7 +46,7 @@ install_wp_playground() {
 	fi
 }
 
-if [ "$ENV_TYPE" == "single" ]; then
+if [ "$ENV_TYPE" = "single" ]; then
 	echo "Setting up single site environment..."
 
 	# Install wp-env if needed
@@ -83,7 +80,7 @@ if [ "$ENV_TYPE" == "single" ]; then
 	echo "Site: http://localhost:8888"
 	echo "Admin login: admin / password"
 
-elif [ "$ENV_TYPE" == "multisite" ]; then
+elif [ "$ENV_TYPE" = "multisite" ]; then
 	echo "Setting up multisite environment..."
 
 	# Install wp-env if needed
@@ -124,7 +121,7 @@ elif [ "$ENV_TYPE" == "multisite" ]; then
 	echo "Test site: http://localhost:8888/testsite"
 	echo "Admin login: admin / password"
 
-elif [ "$ENV_TYPE" == "playground-single" ]; then
+elif [ "$ENV_TYPE" = "playground-single" ]; then
 	echo "Setting up WordPress Playground single site environment..."
 
 	# Install wp-playground if needed
@@ -193,7 +190,7 @@ EOF
 	echo "Admin login: admin / password"
 	echo "Press Ctrl+C to stop the server when done."
 
-elif [ "$ENV_TYPE" == "playground-multisite" ]; then
+elif [ "$ENV_TYPE" = "playground-multisite" ]; then
 	echo "Setting up WordPress Playground multisite environment..."
 
 	# Install wp-playground if needed
@@ -205,6 +202,7 @@ elif [ "$ENV_TYPE" == "playground-multisite" ]; then
 	zip -r dist/plugin.zip . -x "node_modules/*" "dist/*" ".git/*"
 
 	# Update blueprint to use local plugin
+	# shellcheck disable=SC2154
 	cat >playground/multisite-blueprint.json <<EOF
 {
   "landingPage": "/wp-admin/network/",