v1.16.1.0: rebump after queue-collision (PR #1233 took v1.16.0.0)

CI's "Check VERSION is not stale vs queue" job (job 73105686380) failed
with: "VERSION drift: PR #1234 claims v1.15.1.0 but the queue has moved —
next free slot is v1.16.1.0." PR #1233 (garrytan/browserharness) entered
the queue claiming v1.16.0.0 between when this branch's prior /ship ran
and when CI evaluated, so v1.15.1.0 is stale. Rebumping on top.

Files updated:
- VERSION                                                     1.15.1.0 → 1.16.1.0
- package.json                                                1.15.1.0 → 1.16.1.0
- CHANGELOG.md heading + Before/After columns                 1.15.1.0 → 1.16.1.0
- CHANGELOG removal target (consumers.json + config keys)     1.16.0.0 → 1.17.0.0
- gstack-upgrade/migrations/v1.15.1.0.sh                      → renamed v1.16.1.0.sh + log prefix
- bin/gstack-brain-consumer "DEPRECATED in" + "removal in"    1.15.1.0/1.16.0.0 → 1.16.1.0/1.17.0.0
- bin/gstack-brain-uninstall "since vX.Y.Z.W"                 1.15.1.0 → 1.16.1.0
- test/gstack-upgrade-migration-v1_15_1_0.test.ts             → renamed v1_16_1_0.test.ts

No behavior change. 26/26 wireup + migration tests still pass on the rename.
Full bun test suite: exit 0, 0 failures.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gstack",
-  "version": "1.15.1.0",
+  "version": "1.16.1.0",
   "description": "Garry's Stack — Claude Code skills + fast headless browser. One repo, one install, entire AI engineering workflow.",
   "license": "MIT",
   "type": "module",