License under AGPL-3.0-or-later and publish documentation via GitHub Pages

Parts of this repository are derived from canonical/packer-maas, which Canonical
distributes under the AGPLv3, so its copyleft carries over and a permissive or
plain-GPL licence is not available:

  * maas/curtin_userdata_custom.in is adapted from upstream's
    debian/curtin_userdata_custom_amd64, with several late_commands copied
    verbatim (the PXE-disable call, the target bind mount, the cloud.cfg rewrite
    and the zz-update-grub fix)
  * overlay/curtin/curtin-hooks follows upstream's debian/scripts/curtin-hooks:
    same imports, same load_command_environment -> load_command_config ->
    builtin_curthooks -> cleanup structure, near-identical cleanup(). The
    kernel-disabling and interface-pinning functions are original.

The upstream template itself is not vendored; it is cloned at build time and
pinned by PM_REF.

Adds the full AGPL-3.0 text as LICENSE and SPDX-License-Identifier headers to
every source file, placed after the shebang or the #cloud-config marker so both
keep working. deploy-cluster.sh's --help filters the new header lines out of the
usage text it extracts from its own comment block.

GitHub Pages serves index.md, which includes README.md, so the site cannot drift
from the repository documentation. Nothing but build/ is excluded, which keeps
the README's relative links to LICENSE, scripts/ and maas/examples/ resolving on
the published site.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -30,7 +30,8 @@ variable change away (see [Moving to a new Proxmox release](#moving-to-a-new-pro
 - [Repository layout](#repository-layout)
 - [Troubleshooting](#troubleshooting)
 - [Verified status](#verified-status)
-- [Credits and licensing](#credits-and-licensing)
+- [Licensing](#licensing)
+- [References](#references)
 
 ---
 
@@ -808,17 +809,40 @@ Not yet exercised: `PVE_CLUSTER_LINK0` (a separate corosync network), the
 
 ---
 
-## Credits and licensing
+## Licensing
 
-Built on [`canonical/packer-maas`](https://github.com/canonical/packer-maas) (AGPL-3.0),
-which is cloned at build time rather than vendored here.
+**AGPL-3.0-or-later.** See [LICENSE](LICENSE).
 
-References:
+This is not a free choice. The project builds on
+[`canonical/packer-maas`](https://github.com/canonical/packer-maas), which Canonical
+distributes under the AGPLv3, and parts of this repository are derived from it:
+
+- `maas/curtin_userdata_custom.in` is adapted from upstream's
+  `debian/curtin_userdata_custom_amd64`. Several `late_commands` are copied verbatim —
+  the PXE-disable call, the `mount --bind` of the target, the cloud.cfg rewrite and the
+  `zz-update-grub` fix.
+- `overlay/curtin/curtin-hooks` follows upstream's `debian/scripts/curtin-hooks`:
+  the same imports, the same
+  `load_command_environment` → `load_command_config` → `builtin_curthooks` → `cleanup`
+  structure, and a near-identical `cleanup()`. The kernel-disabling and
+  interface-pinning functions are original.
+
+Because those are derivative works, the AGPLv3's copyleft carries over and the project
+cannot be released under a permissive licence, or under the plain GPL.
+
+The upstream template itself is **not vendored**. It is cloned at build time and pinned
+by `PM_REF`; the Makefile applies a small, explicit patch to it.
+
+In practice AGPLv3 asks very little of anyone using this. It is build tooling, not a
+network service: building images, uploading them to MAAS and running the resulting
+Proxmox nodes triggers no obligation. Section 13 — the clause AGPL is known for — only
+applies to someone who offers a *modified version of this software* to others over a
+network.
+
+## References
 
 - [MAAS — Build custom images](https://canonical.com/maas/docs/how-to-build-custom-images)
 - [Proxmox VE — Install on Debian 13](https://pve.proxmox.com/wiki/Install_Proxmox_VE_on_Debian_13_Trixie)
 - [Proxmox VE — Cluster Manager](https://pve.proxmox.com/wiki/Cluster_Manager)
 - [`pvecm(1)`](https://pve.proxmox.com/pve-docs/pvecm.1.html)
-
-No license has been chosen for this repository yet; add one before expecting others to
-reuse it.
+- [`canonical/packer-maas`](https://github.com/canonical/packer-maas)