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>

README.md

@@ -13,7 +13,7 @@ sudo terractl attach calamity
 
 - **Many servers, one host.** Each instance is `terraria@<name>.service`, with its own directory, port, world and mods.
 - **Vanilla and tModLoader side by side**, chosen per instance.
-- **No manual installs.** Drop the official server zips into `/var/lib/terraria/dist`; they're unpacked on demand, and each instance can be pinned to a specific version.
+- **No manual installs.** Drop the official server zips into `/var/lib/terraria/dist`; they're unpacked on demand, and each instance can be pinned to a specific version. `terractl upgrade` checks a new zip, backs up the worlds and switches version in one step.
 - **Safe shutdowns.** Stopping a server saves the world first.
 - **A live console.** `terractl attach` shows the server's output and lets you type commands into it; `terractl watch` just shows the output.
 - **Mod management.** `terractl mods` installs, enables and removes mods, with correct file ownership.