ci: Local bin/relock flow with mise

.devcontainer/run

@@ -22,6 +22,10 @@ if [[ ! -x "$MISE_BIN" ]]; then
   exit 1
 fi
 
+# .mise.toml lists the whole Ruby matrix for bin/relock; the devcontainer profile
+# overrides it to pin the single Ruby baked into the image.
+export MISE_ENV=devcontainer
+
 # Activate mise for this shell so PATH/shims are resolved correctly.
 eval "$("$MISE_BIN" activate bash)"