@flags-sdk/posthog: upgrade posthog-node; explicit evaluation modes (#436)

* @flags-sdk/posthog: upgrade posthog-node

* @flags-sdk/posthog: make local vs remote evaluation explicit

The default adapter passed POSTHOG_PERSONAL_API_KEY into the runtime
posthog-node client, which enabled local evaluation and started a
feature-flag poller in every warm process. On serverless this produced
large, traffic-independent PostHog feature flag request volume.

Local evaluation is now opt-in via POSTHOG_SECRET_KEY; without it the
adapter evaluates remotely. POSTHOG_PERSONAL_API_KEY is used only by
getProviderData (Flags Explorer) and no longer affects runtime
evaluation. Drops the forced 10s poll interval in favor of the v5
default. Updates docs, README, and tests.

* remove unused @vercel/edge-config dependency

* add remote vs local tradeoffs

* rm edge config tag

* modernize

* rm trimKey

* lockfile

* update

* fixes

* reword changeset

* merge changesets

* reword changelog