Show templates on first run and make every template valid #223

Related map: #210

Problem

On the first run, the "New workflow" button opens a blank editor and skips the 18 workflow templates, except in the AI-first variant. The templates also do not work as shipped: they have no sender, five have empty triggers and fail validation, and some use events a new project does not have.

Hints

• Find the first-run empty state and the experiment that decides between the blank editor, the templates and the AI-first path. Respect the running experiment.
• Find where the templates are defined and how they are validated, and check each one.
• Think about what a template should do when the team has no verified sender yet, or lacks the event it triggers on.
• The workflows list v2 work in flight (workflows-list-v2/* branches) and the template link fix (workflows/keep-email-template-link) touch nearby code. Check them first.

Done

A draft PR on PostHog/posthog from the Silthus fork, built test-first with /tdd and handed off by /qa-swarm, with before and after screenshots.