license under MIT

LICENSE (MIT), installed as /usr/share/doc/terraria-server-manager/copyright
in both packages and with %license in the RPM. README gains a License
section and an up-to-date repository layout and build section.

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

AGENTS.md

@@ -30,6 +30,7 @@ the specification of how things behave.
 | `.gitea/workflows/release.yaml` | On a `v*` tag: `make deb rpm` (on Ubuntu), publish the release and the apt and RPM repositories. |
 | `tools/windows/export-tmod-mods.ps1` | Helper for players to export their enabled tModLoader mods. |
 | `docs/guide.md`, `README.md`, `CHANGELOG.md`, `VERSION` | Docs and release metadata. |
+| `LICENSE` | MIT. Installed as `/usr/share/doc/terraria-server-manager/copyright` (and `%license` in the RPM). |
 
 Placeholders `@VERSION@`, `@BINDIR@`, `@DOCDIR@` and `@PKGLIBDIR@` in
 `bin/terractl` and the unit files are filled in by `make install`/`make deb`.