Build strand: the Wizard migrate family (Resend parity) #540

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/wizard
Lane: one build lane, Opus 5.5 high, its own worktree, wall-time cap 4 h (waiting on other strands excluded). Prompt: lanes/wizard.md
Blocked by (strands): skill (slice 14)
Waits on (Harley): none

Slices, in order

• Slice 16: Wizard: turn migrate into a family with migrate statsig and migrate resend. PR: a draft PR on PostHog/wizard main.
• Slice 18: Wizard: capture the migration summary and offer the Resend run on install. PR: a draft PR stacked on 16.

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.