fleetd #511: fix wrong --no-build wording in drain-gate abort, pin jar_id() default

The drain-gate abort message told the operator a rerun "with or without
--no-build" would finish the restart. That is wrong: by the time this
message can fire, stage_built_jar has already moved the jar off $JAR, so
--no-build hits require_no_build_jar's own refusal. Reworded to say the
rerun must NOT use --no-build, and why: the built jar is no longer at the
live path that --no-build requires.

Also added a test pinning jar_id()'s no-argument default (reports $JAR,
the live path) and its explicit-argument behavior (reports that path
instead), per fleetd #511 item 2. Not adding a test for the JAR_STAGED rm
-f at line 578 (fleetd #511 documents it as an equivalent mutant — mvn
clean install deletes target/ on the next line regardless).

scripts/redeploy-fleetd.sh

@@ -615,8 +615,9 @@ if [ -n "$OLD_PID" ] && [ "$ASSUME_YES" = 0 ]; then
     # than before this run started, even though the running daemon itself was never touched.
     if [ "$DO_BUILD" = 1 ] && [ -f "$JAR_STAGED" ]; then
       die "aborted — the running daemon was NOT touched, but the freshly built jar is sitting at
-          $JAR_STAGED, not yet swapped into $JAR. Rerun (with or without --no-build) to finish the
-          restart, or remove $JAR_STAGED by hand if you want to discard this build."
+          $JAR_STAGED, not yet swapped into $JAR. Rerun WITHOUT --no-build to finish the restart —
+          the freshly built jar is no longer at the live path that --no-build requires — or
+          remove $JAR_STAGED by hand if you want to discard this build."
     fi
     die "aborted — nothing changed"
   fi