add upgrade command; reinstall keeps the same version

- terractl upgrade <name> lists the zips an instance can use; with a zip
  or 'latest' it checks the zip, stops the server, backs up worlds and
  configs to instances/.backups/, pins ZIP=, unpacks right away and
  starts the server again. Downgrades need --downgrade.
- reinstall pins the installed zip when ZIP= is blank, so it never
  changes version, and keeps server/.installed (uses a .reinstall flag).
- Resolve terractl's own path before cd /, so a relative $0 works.
- Docs, completion and the daemon's command list; version 0.0.12.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

VERSION

@@ -1 +1 @@
-0.0.11
+0.0.12