chore: bump v1.12.3.0 references to v1.15.1.0 to match merged-with-main release

Internal-only renames after merging origin/main bumped this branch's release
target from v1.12.3.0 → v1.15.1.0:

- gstack-upgrade/migrations/v1.12.3.0.sh → v1.15.1.0.sh (rename + log-prefix
  bump from "[v1.12.3.0]" to "[v1.15.1.0]")
- bin/gstack-brain-consumer header: "DEPRECATED in v1.12.3.0" → "DEPRECATED in
  v1.15.1.0"; removal target bumped from v1.13.0.0 → v1.16.0.0 (next minor
  after v1.15.1.0).
- bin/gstack-brain-uninstall: "no longer written ... since v1.12.3.0" →
  "since v1.15.1.0".

No behavior change. Test suite still 16/16 passing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

bin/gstack-brain-consumer

@@ -1,10 +1,10 @@
 #!/usr/bin/env bash
 # gstack-brain-consumer — manage the consumer (reader) registry.
 #
-# DEPRECATED in v1.12.3.0. This binary targets a gbrain HTTP /ingest-repo
+# DEPRECATED in v1.15.1.0. This binary targets a gbrain HTTP /ingest-repo
 # endpoint that never shipped on the gbrain side. Live federation now uses
 # `gbrain sources` directly via bin/gstack-gbrain-source-wireup. This file
-# stays for one cycle to avoid breaking external scripts; removal in v1.13.0.0.
+# stays for one cycle to avoid breaking external scripts; removal in v1.16.0.0.
 #
 # Consumer = a reader that ingests the gstack-brain git repo as a source of
 # session memory. v1 primary consumer is GBrain; later versions can register