Upgrade note: cookie consent removed, GTM now opt-in via gtm_id alone (vibexp/vibexp#740)

[vibexp-gh]

What

Add an upgrade note under user-guide/self-hosting/upgrading for the cookie-consent removal and the GTM config change shipped by vibexp/vibexp#740 (PR vibexp/vibexp#741), and sweep any page that documents the removed surfaces.

Why

The change is breaking for a specific operator configuration and silently changes behaviour:

• frontend.gtm_enabled (env VITE_GTM_ENABLED) no longer exists. Setting frontend.gtm_id is now the entire opt-in.
• An operator who set gtm_id but never set gtm_enabled: "true" had GTM off; after upgrading, GTM starts loading. Anyone with both set is unaffected.
• A leftover gtm_enabled: key does not fail startup — nested unknown keys are silently ignored by the loader (only removed top-level sections fail fast) — so an operator gets no signal. The note is the only warning they will see.
• VibeXP no longer renders a cookie-consent banner and no longer sets Google Consent Mode defaults. Consent policy is now entirely the operator's, configured in their own GTM container. Operators in consent-required jurisdictions must handle it there.
• Any stored cookieConsent / vx_cookie_consent localStorage entry is deleted on next load.

How

1. Add the upgrade entry under user-guide/self-hosting/upgrading, covering: the removed key, the "gtm_id alone is the opt-in" rule, the behaviour flip above, the removed banner, and the consent-is-yours-now framing.
2. Grep the docs site for gtm_enabled, VITE_GTM_ENABLED, "cookie consent", "consent mode" and update or delete each hit.
3. Check the configuration-reference page's frontend: block matches the shipped config.example.yaml.

Timing: docs track the latest published release, not main — this lands with the release that carries vibexp/vibexp#740, not before.

Acceptance criteria

• Upgrade note published under user-guide/self-hosting/upgrading.
• No gtm_enabled / VITE_GTM_ENABLED reference remains anywhere on the docs site.
• No page describes a cookie-consent banner or Consent Mode behaviour.
• The frontend: config reference matches the shipped config.example.yaml.