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

sshd/server.go

@@ -9,8 +9,9 @@ import (
 	"net"
 	"sync"
 
-	"github.com/armon/go-radix"
 	"golang.org/x/crypto/ssh"
+
+	"github.com/slackhq/nebula/diag"
 )
 
 type SSHServer struct {
@@ -25,10 +26,9 @@ type SSHServer struct {
 	trustedKeys map[string]map[string]bool
 	trustedCAs  []ssh.PublicKey
 
-	// List of available commands
-	helpCommand *Command
-	commands    *radix.Tree
-	listener    net.Listener
+	// The commands this server serves. Shared with every other transport, see diag.Registry.
+	commands *diag.Registry
+	listener net.Listener
 
 	// ctx parents per-Run contexts. Cancelling it (e.g. via Control.Stop) tears the server down even
 	// across reloads, since each Run derives a fresh child rather than reusing this one directly.
@@ -38,11 +38,11 @@ type SSHServer struct {
 // NewSSHServer creates a new ssh server rigged with default commands and prepares to listen.
 // The ssh server's context is parented off the supplied ctx so cancelling it
 // (e.g. on Control.Stop) tears down active sessions and closes the listener.
-func NewSSHServer(ctx context.Context, l *slog.Logger) (*SSHServer, error) {
+func NewSSHServer(ctx context.Context, l *slog.Logger, commands *diag.Registry) (*SSHServer, error) {
 	s := &SSHServer{
 		trustedKeys: make(map[string]map[string]bool),
 		l:           l,
-		commands:    radix.New(),
+		commands:    commands,
 		ctx:         ctx,
 	}
 
@@ -90,14 +90,6 @@ func NewSSHServer(ctx context.Context, l *slog.Logger) (*SSHServer, error) {
 		ServerVersion:     fmt.Sprintf("SSH-2.0-Nebula???"),
 	}
 
-	s.RegisterCommand(&Command{
-		Name:             "help",
-		ShortDescription: "prints available commands or help <command> for specific usage info",
-		Callback: func(a any, args []string, w StringWriter) error {
-			return helpCallback(s.commands, args, w)
-		},
-	})
-
 	return s, nil
 }
 
@@ -160,11 +152,6 @@ func (s *SSHServer) AddAuthorizedKey(user, pubKey string) error {
 	return nil
 }
 
-// RegisterCommand adds a command that can be run by a user, by default only `help` is available
-func (s *SSHServer) RegisterCommand(c *Command) {
-	s.commands.Insert(c.Name, c)
-}
-
 // Run begins listening and accepting connections. Each invocation derives a fresh per-Run context
 // from the constructor-supplied ctx so a Stop+Run sequence (used by config reload) starts clean
 // rather than carrying a permanently-cancelled context across runs.