Build strand: posthog-node (Resend parity) #536

Map: Map: send code-managed email templates with one PostHog call
Spec: Spec: Resend parity, section "Slicing: builders per strand"
Contract: PostHog/posthog#120921
Repository: PostHog/posthog-js
Lane: one build lane, Opus 5.5 high, its own worktree, wall-time cap 6 h (waiting on other strands excluded). Prompt: lanes/posthog-node.md
Blocked by (strands): send-api (slice 10 needs 04; slice 11 needs 08). Slice 09 starts at once.
Waits on (Harley): none

Slices, in order

• Slice 09: posthog-node: emails.send with the template form and the shared client behavior. PR: new commits on feat(node): send transactional email with the flat SDK contract PostHog/posthog-js#5312, or a draft PR stacked on it. Draft: feat(node): send transactional email with the flat SDK contract PostHog/posthog-js#5312.
• Slice 10: posthog-node: the code form, with rendered content and local templates. PR: a draft PR stacked on 09. Draft: feat(node): send email content from code, uploading local templates by hash PostHog/posthog-js#5371.
• Slice 11: posthog-node: read an email's status with emails.get. PR: a draft PR stacked on 10. Draft: feat(node): read an email's status with emails.get PostHog/posthog-js#5372.
• Follow-up: posthog-node: cc, bcc, headers, tags and attachments on the code form, per the contract's 2026-10-22 change. PR: a draft PR stacked on 10, with 11 restacked on it. Draft: feat(node): send cc, bcc, headers, tags and attachments with a code send PostHog/posthog-js#5373.

Done when

• Every slice has its draft PR with its red/green test and owed proofs.
• One GPT-6.1 Sol high review of the strand's whole diff, findings fixed or rejected with a reason.
• The lane's handoff, with its measurements, reached the map conductor.