Expose GDS abilities through WordPress 7 API

[oxyc]

Summary

• Mark GDS MCP abilities as REST-visible via meta.show_in_rest for the WordPress 7 abilities registry.
• Register gds/help through the shared ability helper so it gets the same MCP and REST metadata defaults.
• Bootstrap wordpress/mcp-adapter from gds-mcp when the adapter is installed, removing the host-project MU-plugin requirement.
• Update docs/tests to describe the new setup and assert GDS abilities opt into MCP through their own metadata.
• Update the WP PHPUnit dev constraint to ^7.0.

Verification

• composer lint:fix
• composer lint
• php -l src/Plugin.php
• php -l tests/bootstrap.php
• php -l tests/Integration/SchemaValidationTest.php
• php -l tests/Integration/McpProtocolTest.php
• git diff --check
• composer test could not run locally because the WP PHPUnit bootstrap is missing WP_TESTS_DOMAIN, WP_TESTS_EMAIL, WP_TESTS_TITLE, and WP_PHP_BINARY.