chore: changelog for GST-118 (unreleased)

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Changelog
 
+## [Unreleased]
+
+**gstack no longer leaves Ubicloud VMs running after a lane dies (GST-118).** A runner killed mid-run, a copy of the runner without a TTL, or a VM with no owner could keep billing until someone noticed. Now every VM names its owner and its lifetime, the next `up` anywhere destroys anything past its lifetime, and every run says in one line whether its VM is gone.
+
+### Fixed
+- **`scripts/ubicloud/ubi-runner.sh`**: `up`, `run` and `verify` refuse without `UBI_OWNER` (`UBI_OWNER_REQUIRED`), so no VM is created that nobody can name. The default TTL drops from 4 hours to 3 (`UBI_TTL_HOURS`, 1-999), checked only when a VM is created, so an old setting never blocks `down` or `reap`. `reap`, which every `up` runs first, now also destroys `ubirun-*` VMs that carry no TTL (made by older runner copies) once they are 4 hours old (`UBI_LEGACY_HOURS`); names outside the `ubirun-` prefix are still never touched.
+- **`UBICLOUD: left=<0|1|unknown> owner=<o> vm=<name> others=<n>`**: `run` and `verify` end with this line, checked against the project's VM list after teardown. Anything but `left=0` turns a passing command into exit 1 and prints the `down` command to run; `run --keep` prints `left=1 ... kept=1` and keeps its exit status.
+- **`scripts/ubicloud/pregate-remote.sh`**: the remote pregate refuses without `UBI_OWNER` (`PREGATE_REMOTE_OWNER_REQUIRED`), names its VM with a TTL taken from its own stop deadline plus 15 minutes (`-t1` at today's 2,700-second timeout), and fails with `PREGATE_REMOTE_INFRA` unless the runner reports `left=0`. It reads that line from the file named by `UBI_LEFT_FILE`, never from the remote command's output.
+- docs/TESTING_INTERNALS.md, docs/pregate.md, docs/skills.md and the `/ubicloud` skill describe the owner rule, the new TTL default, the legacy sweep and the `left=` line.
+
+### Tests
+- `test/ubicloud-runner.test.ts`: refusal without an owner, the TTL bounds, the legacy sweep at the default and at `UBI_LEGACY_HOURS=2`, `left=0`, `left=1` on a failed destroy (with a failing command keeping exit 7), `run --keep`, and read-only commands accepting an old `UBI_TTL_HOURS=0`.
+- `test/ubicloud-pregate-remote.test.ts`: the owner refusal, the `-t1` TTL at today's timeout, and `left=1`, `left=unknown` and a missing line each failing closed with the `down` hint.
+
 ## [1.91.335.0] - 2026-10-30
 
 ### Fixed