feat(membershipd): add 'user add/list/revoke' local admin CLI

Local administration surface for the user allowlist, dispatched before the
server flag set parses os.Args. It opens the SQLite store directly with no
network or auth: running on the bus host is trusted by design, which is how
the first admin is seeded (breaking the chicken-egg of needing an admin to
add an admin). Validates that sign-pub is a 32-byte Ed25519 key in hex and
tolerates the sign-pub positional appearing before or after --db.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/membershipd/main.go

@@ -22,6 +22,16 @@ import (
 )
 
 func main() {
+	// Subcommand dispatch: `membershipd user ...` is the local administration CLI
+	// (seed/list/revoke bus users) and must be handled before the server flag set
+	// parses os.Args. Running the CLI on the bus host is trusted by design (whoever
+	// has a shell there already controls the service), which is how the first admin
+	// is seeded without a chicken-egg auth problem.
+	if len(os.Args) > 1 && os.Args[1] == "user" {
+		runUserCLI(os.Args[2:])
+		return
+	}
+
 	var (
 		bind      = flag.String("bind", "127.0.0.1", "network interface to bind the HTTP API and the embedded NATS to; use 0.0.0.0 to accept LAN/remote peers")
 		natsURL   = flag.String("nats-url", "", "external NATS url; empty starts an embedded server")