fix(plugin): drop API-key-derived user_id, restore $USER fallback

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.

mem0-plugin/scripts/on_session_start.sh

@@ -36,7 +36,7 @@ echo "## Mem0 Identity"
 echo ""
 echo "Active user_id: \`$MEM0_RESOLVED_USER_ID\`"
 echo ""
-echo "Always include \`{\"user_id\": \"$MEM0_RESOLVED_USER_ID\"}\` (wrapped in an \`AND\` clause) in every \`search_memories\` filter and as \`user_id\` on every \`add_memory\` call. This keeps memories under one bucket regardless of which machine you're on."
+echo "Always include \`{\"user_id\": \"$MEM0_RESOLVED_USER_ID\"}\` (wrapped in an \`AND\` clause) in every \`search_memories\` filter and as \`user_id\` on every \`add_memory\` call. This keeps the agent's MCP calls aligned with the bucket the hooks write to."
 echo ""
 
 if [ "$SOURCE" = "startup" ]; then