fix: [nomad-step-0] S0.1-fix — bin/disinto swallows --backend=nomad as repo_url positional (#835)

Why: disinto_init() consumed $1 as repo_url before the argparse loop ran,
so `disinto init --backend=nomad --empty` had --backend=nomad swallowed
into repo_url, backend stayed at its "docker" default, and the --empty
validation then produced the nonsense "--empty is only valid with
--backend=nomad" error — flagged during S0.1 end-to-end verification on
a fresh LXC. nomad backend takes no positional anyway; the LXC already
has the repo cloned by the operator.

Change: only consume $1 as repo_url if it doesn't start with "--", then
defer the "repo URL required" check to after argparse (so the docker
path still errors with a helpful message on a missing positional, not
"Unknown option: --backend=docker").

Verified acceptance criteria:
  1. init --backend=nomad --empty             → dispatches to nomad
  2. init --backend=nomad --empty --dry-run   → 9-step plan, exit 0
  3. init <repo-url>                          → docker path unchanged
  4. init                                     → "repo URL required"
  5. init --backend=docker                    → "repo URL required"
                                                (not "Unknown option")
  6. shellcheck clean

Tests: 4 new regression cases in tests/disinto-init-nomad.bats covering
flag-first nomad invocation (both --flag=value and --flag value forms),
no-args docker default, and --backend=docker missing-positional error
path. Full suite: 10/10 pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/disinto

@@ -707,13 +707,18 @@ _disinto_init_nomad() {
 }
 
 disinto_init() {
-  local repo_url="${1:-}"
-  if [ -z "$repo_url" ]; then
-    echo "Error: repo URL required" >&2
-    echo "Usage: disinto init <repo-url>" >&2
-    exit 1
+  # Only consume $1 as repo_url if it looks like a positional arg (not a
+  # flag). The nomad backend (#835) takes no positional — the LXC already
+  # has the repo cloned by the operator, and repo_url is a docker-backend
+  # concept. Eagerly consuming `--backend=nomad` as repo_url produced the
+  # nonsense "--empty is only valid with --backend=nomad" error seen in
+  # S0.1 end-to-end testing on a fresh LXC. Defer the "repo URL required"
+  # check to after argparse, where we know the backend.
+  local repo_url=""
+  if [ $# -gt 0 ] && [[ "$1" != --* ]]; then
+    repo_url="$1"
+    shift
   fi
-  shift
 
   # Parse flags
   local branch="" repo_root="" ci_id="0" auto_yes=false forge_url_flag="" bare=false rotate_tokens=false use_build=false dry_run=false backend="docker" empty=false
@@ -741,6 +746,16 @@ disinto_init() {
     *) echo "Error: invalid --backend value '${backend}' (expected: docker|nomad)" >&2; exit 1 ;;
   esac
 
+  # Docker backend requires a repo_url positional; nomad doesn't use one.
+  # This check must run *after* argparse so `--backend=docker` (with no
+  # positional) errors with a helpful message instead of the misleading
+  # "Unknown option: --backend=docker".
+  if [ "$backend" = "docker" ] && [ -z "$repo_url" ]; then
+    echo "Error: repo URL required" >&2
+    echo "Usage: disinto init <repo-url> [options]" >&2
+    exit 1
+  fi
+
   # --empty is nomad-only today (the docker path has no concept of an
   # "empty cluster"). Reject explicitly rather than letting it silently
   # do nothing on --backend=docker.