Build strand: the Migrate from Resend page (Resend parity) #541

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.com
Lane: one build lane, Opus 5.5 high, its own worktree, wall-time cap 2 h (waiting on other strands excluded). Prompt: lanes/docs-migrate.md
Blocked by (strands): skill (slice 15), wizard (slice 16)
Waits on (Harley): none

Slices, in order

• Slice 19: Docs: Migrate from Resend, with a prompt to copy into your agent. PR: one draft PR on master.

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.