fix: make the in-process execution mode explicit in specs and contract checks

Two coupled holes surfaced in the Phase 0-3 review. duet--transfer-spec copied only :argv, :default-directory, and :process-environment out of a backend's command result, dropping the :tramp marker, so a TRAMP-routed spec arrived as :argv nil with nothing telling the executor to copy in process. And duet--check-command leaned on listp, where nil is a list in Elisp, so a command builder returning nil or a bare :argv nil passed the minimum tier.

Both turn on the same idea, so they share a fix. duet--command-spec-executable-p defines a runnable spec: a non-empty plist with either a non-empty :argv of strings (a CLI backend) or a declared in-process mode such as :tramp. The contract checker rejects anything else, and transfer-spec now carries :tramp through, so the TRAMP fallback has a positive execution signal rather than an ambiguous nil argv.

The legitimate TRAMP backend keeps passing because it declares its mode. A broken backend that forgets argv no longer slips through.

1 files changed, 26 insertions, 0 deletions

diff --git a/tests/test-duet-backend.el b/tests/test-duet-backend.el
index b1d9c34..f0a999c 100644
--- a/tests/test-duet-backend.el
+++ b/tests/test-duet-backend.el
@@ -211,6 +211,32 @@ value for a duplicated keyword."
                                                (list :shell-command "rm -rf /")))))
     (should (duet-backend-check-minimum b))))
 
+(ert-deftest test-duet-backend-check-minimum-flags-nil-command-spec ()
+  "A command builder returning nil fails the minimum tier (nil is a list)."
+  (let ((b (test-duet-backend--fake 'nilcmd 10 :command (lambda (_s _d _o) nil))))
+    (should (duet-backend-check-minimum b))))
+
+(ert-deftest test-duet-backend-check-minimum-flags-empty-argv-cli ()
+  "A CLI backend with nil argv and no declared in-process mode fails."
+  (let ((b (test-duet-backend--fake
+            'noargv 10
+            :command (lambda (_s _d _o) (list :argv nil :default-directory "/")))))
+    (should (duet-backend-check-minimum b))))
+
+(ert-deftest test-duet-backend-check-minimum-accepts-in-process-spec ()
+  "A backend declaring an in-process mode (:tramp) passes with a nil argv."
+  (let ((b (test-duet-backend--fake
+            'inproc 10
+            :command (lambda (_s _d _o) (list :argv nil :tramp t)))))
+    (should (null (duet-backend-check-minimum b)))))
+
+(ert-deftest test-duet-backend-check-minimum-flags-non-string-argv ()
+  "An argv carrying non-string elements is not a runnable CLI command."
+  (let ((b (test-duet-backend--fake
+            'bad 10
+            :command (lambda (_s _d _o) (list :argv '("rsync" 42))))))
+    (should (duet-backend-check-minimum b))))
+
 (ert-deftest test-duet-backend-check-publishable-flags-missing-cleanup ()
   "The publishable tier additionally requires declared cleanup semantics."
   (let ((b (test-duet-backend--fake 'pub 10