feat(#607): rebuild get-shit-done-cc → gsd-core migration (per-package cache + installer auto-cleanup + --dry-run) (#611)

* feat(#607): rebuild get-shit-done-cc → gsd-core migration

Leftover get-shit-done-cc installs poisoned the shared update cache,
causing a permanent false "update available". Rebuild the migration so a
stale old install is both harmless and actively removed.

- Per-package update cache filename (gsd-update-check-<slug>.json) in the
  shared ~/.cache/gsd dir, single-sourced via package-identity; writers
  stamp package_name and readers reject foreign/absent lineage. Multi-
  runtime visibility preserved (same shared dir + filename across runtimes).
- New get-shit-done/bin/lib/legacy-cleanup.cjs seam: detects code-file
  references to the old package + the legacy fixed-name cache across home
  runtime dirs; installer auto-cleans on every install; --dry-run previews
  and mutates nothing. User hooks and dev-preferences are never touched.
- update.md cache-clear globs gsd-update-check*.json across ALL supported
  runtimes (adds cursor/windsurf/augment/trae/qwen/hermes/codebuddy/cline).
- Fix worker MODULE_NOT_FOUND post-install (ship managed-hooks-registry.cjs
  + degrade gracefully) so the per-package cache is always written.
- Diataxis how-to: docs/cleanup-get-shit-done-cc.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#607): add changeset for PR #611

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(#607): set USERPROFILE alongside HOME in dry-run install test for Windows

os.homedir() reads USERPROFILE on win32, so HOME-only isolation let the
spawned installer scan the real runner home on windows-latest. Set both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/README.md

@@ -32,6 +32,7 @@ Language versions: [English](README.md) · [Português (pt-BR)](pt-BR/README.md)
 - [Drive GSD from a tracker issue](how-to/drive-gsd-from-a-tracker-issue.md) — start a phase from a GitHub, Linear, or Jira issue
 - [Migrate from GSD 2](how-to/migrate-from-gsd-2.md) — upgrade an existing GSD 2 project to GSD Core
 - [Update GSD](how-to/update-gsd.md) — re-run the installer to pick up the latest release
+- [Clean up get-shit-done-cc](cleanup-get-shit-done-cc.md) — remove leftover old-package artifacts that cause a spurious `⬆ /gsd:update` indicator after migrating to `@opengsd/gsd-core`
 - [Recover and troubleshoot](how-to/recover-and-troubleshoot.md) — fix common problems, rebuild context, and uninstall
 
 ---