fix(ci): restructure CI for PHP 8.1+ and proper PHPCS setup

- Drop PHP 8.0 from matrix — DTOs use readonly properties (8.1+)
- Add composer.json with PHPCS/WPCS as dev dependencies
- Add phpcs.xml.dist for WordPress coding standard config
- Use dealerdirect/phpcodesniffer-composer-installer for auto path setup
- Separate lint jobs: PHP syntax (8.1/8.2/8.3) + PHPCS + file-size
- Update Requires PHP header to 8.1
- Update ARCHITECTURE.md file tree

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

phpcs.xml.dist

@@ -0,0 +1,17 @@
+<?xml version="1.0"?>
+<ruleset name="Peptide Repo Core">
+	<description>WordPress coding standards for Peptide Repo Core.</description>
+
+	<file>.</file>
+
+	<exclude-pattern>/vendor/*</exclude-pattern>
+	<exclude-pattern>/node_modules/*</exclude-pattern>
+	<exclude-pattern>/tests/*</exclude-pattern>
+
+	<arg name="extensions" value="php"/>
+	<arg name="severity" value="1"/>
+	<arg name="warning-severity" value="0"/>
+	<arg value="sp"/>
+
+	<rule ref="WordPress"/>
+</ruleset>