feat(sendgrid): transactional email integration (#18)

[jackulau]

Closes #18.

Adds a SendGrid transactional email integration to @jam-nodes/nodes with three new nodes, an API-key credential, full Zod schemas, and a comprehensive Vitest suite. Follows the established Slack integration pattern (packages/nodes/src/integrations/slack/) end-to-end.

What changed

New files

• packages/nodes/src/integrations/sendgrid/credentials.ts — defineApiKeyCredential with header auth Authorization: Bearer {{apiKey}}.
• packages/nodes/src/integrations/sendgrid/schemas.ts — input/output Zod schemas for all three operations, plus shared SendgridContentSchema, SendgridAttachmentSchema, SendgridContactSchema.
• packages/nodes/src/integrations/sendgrid/sendgrid-send-email.ts — sendgridSendEmailNode (POST /v3/mail/send).
• packages/nodes/src/integrations/sendgrid/sendgrid-create-contact.ts — sendgridCreateContactNode (PUT /v3/marketing/contacts, upsert).
• packages/nodes/src/integrations/sendgrid/sendgrid-get-contacts.ts — sendgridGetContactsNode (POST /v3/marketing/contacts/search, SGQL).
• packages/nodes/src/integrations/sendgrid/index.ts — barrel export.
• packages/nodes/src/integrations/sendgrid/__tests__/sendgrid.test.ts — 54 unit tests.

Modified files

• packages/core/src/types/node.ts — adds sendgrid?: { apiKey: string } to NodeCredentials.
• packages/nodes/src/integrations/index.ts — re-exports the new nodes, schemas, types, and credential.

Design notes

• Pure nodes — every executor is a thin wrapper over fetchWithRetry; retry / cache / timeout cross-cutting concerns stay in the execution engine per the architectural rule in CLAUDE.md (Issue feat: add execution engine with configurable retry, caching, and timeout #37, PR feat: add an execution engine with configurable retry, caching, and timeout #39).
• Bearer header auth via defineApiKeyCredential, mirroring devto/credentials.ts.
• x-message-id header is the only signal SendGrid returns from /v3/mail/send (the body is empty on 202); the executor reads it from response headers and falls back to an empty string when absent.
• PUT /v3/marketing/contacts is SendGrid's upsert endpoint — the executor surfaces the returned job_id as contactJobId so callers can poll job status.
• POST /v3/marketing/contacts/search is the only contacts endpoint that supports listId filtering. The executor builds an SGQL query with single-quote and backslash escaping to prevent SGQL injection. Results are capped at 50 per call by SendGrid; limit/offset are applied client-side and total reflects contact_count from the response.
• templateId strips top-level content before sending (SendGrid rejects requests that include both).
• Zod superRefine rejects dynamicTemplateData without templateId so callers get an immediate validation error instead of a silent drop.
• subject blocks \\r / \\n as a defense-in-depth guard against header injection.
• All email fields are trimmed before .email() validation so callers don't get opaque 400s from SendGrid for trailing whitespace.

Verification

npm test --workspace=@jam-nodes/nodes      # 249 / 249 (was 195 baseline, +54 SendGrid)
npm run typecheck --workspace=@jam-nodes/core   # exit 0

The pre-existing typecheck errors in slack-send-message.ts / slack-update-message.ts / apify / google-sheets on `main` are unchanged — fixing them is out of scope for this issue.

Acceptance criteria (from #18)

• Credential definition
• All 3 operations
• Template support
• Attachment handling
• Zod schemas
• Unit tests