docs: rework Phase 12 — hub minimization deferred by operator decision

Operator decisions (tmp-decisions.md D1/D2/D4):
  - the hub keeps serving the web UI (zero-install path stays)
  - a native desktop client is offered ALONGSIDE it, not as a replacement
  - hub minimization is off the critical path and may be dropped

Phase 12 was "Hub minimization: registrar and nothing more". Most of it is
dropped: route-inventory blindness test, opaque private-group metadata,
chat_notify metadata minimization, residual schema cleanup. The swarm item
already shipped in 11.5.18.

Two items are kept, because the decision makes them more relevant rather than
less — the hub stays in the trusted path by choice, so what it can substitute
and what code it serves both still matter:

  12.1 key transparency + safety numbers [H3]. This is the last open High
       finding and nothing else fixes it: the hub is the public key directory,
       so substituting a key during an invite hands it the group key silently,
       with no JWT forgery and no code injection. Dropping Phase 12 wholesale
       would have left it open indefinitely.
  12.2 served-SPA integrity: CSP, SRI, and a hub-published signed digest of the
       bundle so a native client can verify what the browser was given.
  12.3 honest labelling of /app/ as the hub-served path.
  12.4 written threat model — the thing that stops the overclaiming pattern.

Recorded consequence: T3 is now accepted permanently for browser users. A hub
that serves the code can exfiltrate keys from the page whatever the protocol
does. The claim that still holds, and that the docs should make, is "the hub
cannot read your content unless it actively attacks you" — not "unreadable by
other parties, even the hub".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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