The deterministic resolver from #5076 silently rebucketed every user
who hadn't set MEM0_USER_ID. With MEM0_API_KEY present (the normal
case — hooks early-exit without it), the resolver picked
"mem0-<sha256(api_key)[:12]>" instead of $USER, so an existing
"deshraj" bucket suddenly became "mem0-f34dd1cba657" on update with
no migration. The old memories aren't reachable from the plugin
anymore, and the user gets a fresh empty bucket without warning.
Resolution priority is now back to:
1. MEM0_USER_ID env var
2. \$USER, else "default"
The ~/.mem0/identity.json cache is no longer written or read; any
existing cache file becomes inert. Users on multiple machines who
want a single bucket can still set MEM0_USER_ID explicitly — that's
the supported path, not silent derivation.
Also dropped the "regardless of which machine you're on" line from
the session bootstrap header since it no longer applies.
Mgeeeek