feat(plugin): add official Resend email provider plugin

[maikunari]

What does this PR do?

Adds an official Resend email provider plugin (@emdash-cms/plugin-resend) that integrates with the EmDash plugin system to provide outbound email delivery via Resend.

Key implementation details:

• Implements the email:deliver exclusive hook for sending emails through the Resend API
• Requires email:provide and network:fetch capabilities
• Uses the webhook-notifier sandboxed plugin architecture (descriptor + sandbox entry)
• Includes an admin settings page for configuring the Resend API key and From address
• Stores credentials securely via the plugin KV store

Discussion: #245

Type of change

• Bug fix
• Feature (requires approved Discussion)
• Refactor (no behavior change)
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes
• pnpm --silent lint:json | jq '.diagnostics | length' returns 0
• pnpm test passes (or targeted tests for my change)
• pnpm format has been run
• I have added/updated tests for my changes (if applicable)
• I have added a changeset (if this PR changes a published package)
• New features link to an approved Discussion: Email Provider Plugins Ecosystem (SMTP, Resend, Postmark, etc.) #245

AI-generated code disclosure

• This PR includes AI-generated code

Screenshots / test output

N/A — plugin has no visual UI beyond the admin settings page rendered via the plugin block system.

[changeset-bot]

🦋 Changeset detected

Latest commit: c0651fe

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@emdash-cms/plugin-resend	Minor

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[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[maikunari]

I have read the CLA Document and I hereby sign the CLA

[maikunari]

recheck

[ascorbic]

I'm going to hold off on merging any other plugins into core for now

[maikunari]

Makes total sense! I'll close this out and publish it as a standalone community plugin instead. Thanks for the quick review and the context on the repo's direction!