The previous gate compared session.user_peer_id against a fresh
_resolve_user_peer_id() call on the same manager. Both values come from
the same resolver with the same inputs, so a non-owner triggering a new
session in a shared channel passed the check and received the owner's
MEMORY.md/USER.md under their peer.
The owner is now a config fact: _declared_owner_peer_id() returns the
sanitized peerName, and migration runs only when the session's user peer
is that peer. Without a declared peerName, migration runs only when no
runtime gateway identity is present (the single-operator CLI path).
Aliases still work: a platform ID mapped onto peerName resolves to the
owner peer before the comparison.
Tests now derive each session's user peer from the real resolver instead
of hand-picking mismatched ids, so the non-owner test fails against the
old gate.
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