Brings PostHog, GA4, HubSpot, Reo.dev, Scarf, and RB2B into shell-docs
with parity to docs/. Adds the client-side PostHog provider with
session-stitched bootstrap and pageview capture, the AnalyticsClient
wrapper that mounts RB2B + GA4 hooks behind a single client boundary,
the Scarf pixel for OSS attribution, and the HubSpot and Reo.dev
scripts.
Renames POSTHOG_PROJECT_KEY to POSTHOG_KEY across shell and shell-docs
middlewares so the env names match the upstream pattern, and env-drives
POSTHOG_HOST with eu.i.posthog.com as the fallback.
Sam Julien