feat(integration): add email channel integration

[realcodesiman]

Summary

• Add new IntegrationEmail channel with SMTP settings (provider: gmail/outlook/other, host, port, username, password, fromAddress)
• Full-stack scaffolding: integration package (integrations/email/), database schema + relations, builder feature (CRUD actions, manage table UI, create form, settings parallel route), and worker registration
• Add email to channelTypes and integrationTypes Zod enums, update all Record<ChannelType, ...> usages
• Update integration-channel SKILL.md with comprehensive step-by-step creation guide, post-creation verification checklist, and common build error reference table

Changes

New files

• integrations/email/ — integration package (schema, handlers, integration definition)
• packages/database/src/schema/integration-email.ts — DB table definition
• packages/database/src/relations/integration-email.ts — Drizzle relations
• apps/builder/src/features/integration-email/ — builder feature (actions, queries, schemas, components, manage page)
• apps/builder/src/app/space/[workspaceId]/(settings)/settings/channels/@email/page.tsx — settings parallel route

Modified files

• packages/database/src/partials/channel.ts — add email to enum
• packages/database/src/partials/integration.ts — add email to enum
• packages/database/src/schema/index.ts — export new schema
• packages/database/src/relations/index.ts — import + spread new relations
• packages/database/src/types.ts — add IntegrationEmailModel type
• apps/builder/src/integration.ts — register email integration
• apps/worker/src/services/integrations.ts — register email integration
• apps/builder/package.json + apps/worker/package.json — add workspace dependency
• apps/builder/.../channels/layout.tsx — add email parallel route slot
• apps/builder/.../inbox-icon.tsx — add MailIcon for email channel
• apps/builder/.../inbox-card-list.tsx — add email entry to cardConfigs
• .agents/skills/integration-channel/SKILL.md — comprehensive rewrite

Test plan

• Run pnpm turbo build — verified passing
• Verify email channel appears in Settings > Channels accordion
• Test create email integration flow via the form
• Test delete email integration via disconnect button
• Verify InboxIcon renders correctly for email channel

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