bin/gstack-update-check: pure bash, checks VERSION against remote once/day,
outputs UPGRADE_AVAILABLE or JUST_UPGRADED. Uses ~/.gstack/ for state.
gstack-upgrade/SKILL.md: new skill with inline upgrade flow for all preambles.
Detects global-git, local-git, vendored installs. Shows What's New from CHANGELOG.
browse/test/gstack-update-check.test.ts: 10 test cases covering all branch paths.
Garry Tan