MailChannels

Clarify best-effort project support through GitHub issues and service support through MailChannels.
Add a MailChannels mailer adapter for Craft CMS 5.11.4 and newer 5.x versions.
Keep API keys in server environment variables and persist only variable names.
Preserve SDK message conversion with synchronous acceptance checks, envelope filtering and sanitized errors.
Add offline contract tests and an isolated installed-Craft smoke test.

Version 1.0.0