fix(#1408): legacy-socket launch guard + seat launch.sh preference (#1409)

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

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

@@ -304,6 +304,17 @@ if [ "$MODE" = stop ]; then
   exit 0
 fi
 
+# #1408 hazard: a seat still living on the DEFAULT tmux socket is invisible to the
+# declared-socket guard below, and launching over it creates a same-name duplicate that
+# name-addressed comms delivery cannot tell apart. Refuse with a distinct code (76,
+# after 75 broker-absent) so a cutover wave script can branch on "seat still on legacy
+# socket" vs "already running" (0) vs "broker absent" (75). Stopping the legacy session
+# belongs to the cutover procedure, never to this launcher.
+if [ -n "$MOSAIC_TMUX_SOCKET" ] && tmux has-session -t "=${AGENT_NAME}" 2>/dev/null; then
+  echo "[fleet] FAIL_LAUNCH seat-on-legacy-socket: session '${AGENT_NAME}' exists on the DEFAULT tmux socket; stop it before launching on '${MOSAIC_TMUX_SOCKET}'." >&2
+  exit 76
+fi
+
 if _tmux has-session -t "=${AGENT_NAME}:0.0" 2>/dev/null; then
   echo "Mosaic agent session already running: $AGENT_NAME on socket ${MOSAIC_TMUX_SOCKET:-(default)}"
   exit 0
@@ -421,9 +432,22 @@ if [ "$MOSAIC_AGENT_RUNTIME" = claude ]; then
     echo "WARNING: could not pre-trust workdir for claude agent $AGENT_NAME" >&2
 fi
 
-LAUNCH_COMMAND=(mosaic yolo "$MOSAIC_AGENT_RUNTIME")
-if [ -n "$MOSAIC_AGENT_MODEL" ]; then LAUNCH_COMMAND+=(--model "$MOSAIC_AGENT_MODEL"); fi
-if [ -n "$MOSAIC_AGENT_REASONING" ]; then LAUNCH_COMMAND+=(--thinking "$MOSAIC_AGENT_REASONING"); fi
+# #1408 hazard: prefer the seat's own launch.sh when the brain provides one. It is the
+# path that binds the auth profile (CLAUDE_SECURESTORAGE_CONFIG_DIR) and seeds the seat
+# config; `mosaic yolo` relocates CLAUDE_CONFIG_DIR to the seat dir (launch.ts
+# activeSeatDir/harnessEnv) but performs neither, so a yolo-launched seat points its
+# config at a directory holding no credentials. The env -i allowlist below still
+# applies: launch.sh reads its own launch.env.
+SEAT_LAUNCH="${BRAIN_HOME}/fleet/agents/${AGENT_NAME}/launch.sh"
+if [ -x "$SEAT_LAUNCH" ]; then
+  LAUNCH_COMMAND=("$SEAT_LAUNCH")
+  echo "[fleet] launch path: seat launch.sh ($SEAT_LAUNCH)"
+else
+  LAUNCH_COMMAND=(mosaic yolo "$MOSAIC_AGENT_RUNTIME")
+  if [ -n "$MOSAIC_AGENT_MODEL" ]; then LAUNCH_COMMAND+=(--model "$MOSAIC_AGENT_MODEL"); fi
+  if [ -n "$MOSAIC_AGENT_REASONING" ]; then LAUNCH_COMMAND+=(--thinking "$MOSAIC_AGENT_REASONING"); fi
+  echo "[fleet] launch path: mosaic yolo (no executable seat launch.sh)"
+fi
 
 # The tmux holder owns a named server. Explicitly clear the pane environment
 # so server/session variables cannot cross the launch boundary; retain only