docs: Add box installation procedure

Canonical, agent-agnostic procedure for bringing a new box into the
flake, from a booted installer through to a deployable system, plus a
thin Claude Code skill pointing at it -- same split as the nixpkgs
upgrade procedure.

Records the conventions that were not written down anywhere: sgdisk
plus an LVM PV for the nix and persist volumes, adopting the
installer's SSH host keys so secrets can be encrypted before first
boot, and taking whatever show-hw-config emits that the flake's own
modules do not already set.

Also notes in AGENTS.md that a changed recipient list should be
re-encrypted per file with ragenix --rekey-one; --rekey rewrites every
secret in secrets/ and buries the actual change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/misc/installer.md

@@ -34,8 +34,10 @@ The custom NixOS installer image used to bootstrap new boxes.
 
 ## Installing a box
 
-The devshell's installer commands ([`devshell/install.nix`](../../devshell/install.nix)) drive
-an install over SSH against a booted installer reachable at `$INSTALLER`:
+The end-to-end procedure — hardware inventory, partitioning, writing the box config, installing and
+documenting it — is in [`install-box.md`](../install-box.md). The devshell's installer commands
+([`devshell/install.nix`](../../devshell/install.nix)) drive an install over SSH against a booted
+installer reachable at `$INSTALLER`:
 
 - `installer-shell` — get a shell on the installer.
 - `do-install <system>` — builds the system's toplevel, `nix copy`s the closure to the