Add nebula ctl, a local socket for the debug commands

Every diagnostic command nebula has was reachable through exactly one door:
the built-in ssh debug server. That server is off by default, and turning it
on means generating a host key, writing an sshd block with authorized public
keys, and SIGHUPing the daemon. That is a lot of ceremony to answer "what
version is this node running".

Nebula now serves the same commands over a local unix socket, enabled by
default, and `nebula ctl <command>` runs them. The socket lives in a 0700
directory so filesystem permissions are the access control; no keys, nothing
on the network. Failing to create it is logged and never blocks startup.

The command registry was already transport neutral, so this is mostly new
transport rather than new commands:

  - diag/ holds the registry, dispatch, writer and wire protocol, moved out
    of sshd because none of it was ever about ssh. sshd and ctl.go dispatch
    against one shared registry.
  - commands.go holds every command implementation, moved out of ssh.go
    (which was 85% not ssh) and renamed off the ssh prefix. Adding a command
    there makes it available over both transports.
  - ssh.go keeps only host keys, authorized users, and the listen address.
  - ctl.go supervises the socket, following the statsServer lifecycle shape.

The wire protocol frames the response rather than terminating it, because
print-cert -raw and list-hostmap -json both emit arbitrary bytes that no
sentinel could safely delimit. argv travels as a list so quoting survives.
Exit statuses are real: 0, 2 for usage, 127 for an unknown command.

Two things fall out. The ssh console now reports a real exit status instead
of a hardcoded zero, so `ssh host list-hostmap` is scriptable too. And eight
command callbacks that silently returned nil on a flags type mismatch now
report it, which the exit status makes visible.

Windows is a stub returning a clear "not supported" until it gets a named
pipe with a security descriptor; iOS and Android are never enabled, having no
daemon for a CLI to attach to.

Breaking for embedders of the sshd package: NewSSHServer takes a
*diag.Registry, SSHServer.RegisterCommand is gone in favor of registering on
that registry, and the command types live in diag rather than sshd.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014fya5fTXGiwX72FUmoL9y3

CHANGELOG.md

@@ -7,6 +7,28 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- New `nebula ctl <command>` subcommand, which runs any of the debug and administrative commands the sshd
+  block exposes without requiring an ssh server, a host key, or authorized keys. Nebula serves them over a
+  local unix socket, configured by the new `ctl` block and enabled by default at `/run/nebula/ctl.sock` on
+  Linux and `/var/run/nebula/ctl.sock` elsewhere. The socket lives in a `0700` directory so filesystem
+  permissions are the access control; failing to create it is logged and never prevents nebula from
+  starting. Packagers running nebula under systemd will want `RuntimeDirectory=nebula` in the unit so the
+  directory exists with the right ownership. Not supported on Windows yet, and never enabled on iOS or
+  Android. Reloadable.
+
+### Changed
+
+- The ssh console now reports a real exit status for `ssh <host> <command>` rather than always reporting
+  success, so commands run that way are scriptable.
+- The debug and administrative commands moved out of `ssh.go` into `commands.go` and are no longer tied to
+  ssh: both the ssh console and `nebula ctl` dispatch against one shared registry, so a command added in
+  one place is available over both. Embedders of the `sshd` package are affected: `sshd.NewSSHServer` now
+  takes a `*diag.Registry`, `sshd.SSHServer.RegisterCommand` is gone in favor of registering on that
+  registry directly, and the command types now live in the `diag` package rather than being re-exported
+  from `sshd`.
+
 ## [1.11.1] - 2026-08-21
 
 See the [v1.11.1](https://github.com/slackhq/nebula/milestone/30?closed=1) milestone for a complete list of changes.