Merge pull request 'fix(fleet): fail the agent launcher when the pane cannot survive (#1241)' (#1244) from fix/1241-launch-failure-visible into next

Reviewed by scooby via git comms (no mosaicstack principal on fomo-lin; review is the comms record, terminal ACK 32d986). Merge directed by Jason 2026-08-16. Part of the five-PR greenfield composite verified E2E on two independent bare boxes.

packages/mosaic/framework/systemd/user/test-fleet-units.sh

@@ -128,6 +128,14 @@ EOF
 sleep 30
 EOF
   chmod 700 "$AGENT_BIN/mosaic"
+  # The launcher resolves the roster's runtime against PANE_PATH before it
+  # spawns anything (#1241), so the runtime this projection names has to be
+  # present here even though the fake `mosaic` above never execs it.
+  cat > "$AGENT_BIN/pi" <<'EOF'
+#!/bin/sh
+sleep 30
+EOF
+  chmod 700 "$AGENT_BIN/pi"
   server_environment_before=$(tmux -L "$TEST_SOCKET" show-environment -g | sort)
   server_sessions_before=$(tmux -L "$TEST_SOCKET" list-sessions | sort)
   if /usr/bin/env -i HOME="$HOLDER_HOME" PATH=/usr/bin:/bin MOSAIC_HOME="$AGENT_HOME" \