fix(fleet): lease-broker activation, symlink-safe unit placement, named launch refusal — Wall 6 (#1292) (#1297)

Co-authored-by: fargo <[email protected]>

packages/mosaic/framework/tools/fleet/start-agent-session.sh

@@ -257,8 +257,36 @@ assert_owned_tmux_server() {
     fail "tmux server ownership or environment validation failed"
 }
 
-# Validate exact server ownership before querying, cleaning, or creating any
-# managed session. An unmanaged or contaminated named socket is never repaired.
+# Lease-broker socket preflight (#1292). The gated runtime (`mosaic yolo …` →
+# launch-runtime.py) registers with the broker or dies ~4 seconds in, with the
+# diagnostic invisible because tmux destroys the dead pane. This check runs
+# BEFORE any tmux effect — including the ownership probe below — so a host
+# without a broker produces a named, surviving refusal instead of a doomed
+# pane. Exit 75 (EX_TEMPFAIL), distinct from 64 (bad projection) and 69 (host
+# not ready for other reasons); the agent@ unit is Type=oneshot with no
+# Restart=, so the failed unit keeps its message instead of looping. Socket
+# resolution matches launch.ts's defaultLeaseBrokerSocket precedence exactly.
+# This preflight DETECTS and REFUSES — it never starts the broker (activation
+# belongs to the fleet control plane; a component that both detects and fixes
+# cannot be used to measure whether the fix worked).
+broker_socket_path() {
+  if [ -n "${MOSAIC_LEASE_BROKER_SOCKET:-}" ]; then
+    printf '%s\n' "$MOSAIC_LEASE_BROKER_SOCKET"
+    return 0
+  fi
+  local runtime_dir="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}"
+  printf '%s\n' "${runtime_dir}/mosaic-lease/broker.sock"
+}
+
+if [ "$MODE" = "launch" ]; then
+  _broker_socket=$(broker_socket_path)
+  if [ ! -S "$_broker_socket" ]; then
+    echo "[fleet] FAIL_LAUNCH broker-absent: lease broker socket ${_broker_socket} missing; runtime launch denied (#1292)." >&2
+    echo "[fleet] remedy: systemctl --user enable --now mosaic-lease-broker.service  (or reinstall via: mosaic fleet install)" >&2
+    exit 75
+  fi
+fi
+
 assert_owned_tmux_server
 
 if [ "$MODE" = interaction ]; then