fix(fleet): consume model_hint + fix socket-default trap (stand-up fixes) (#627)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

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

@@ -2,8 +2,12 @@
 set -euo pipefail
 
 AGENT_NAME=${1:-${MOSAIC_AGENT_NAME:-}}
-MOSAIC_TMUX_SOCKET=${MOSAIC_TMUX_SOCKET:-mosaic-factory}
+# Absent socket ⇒ the LITERAL default tmux socket (no -L). The roster's
+# socket_name is honored when set; absent never silently becomes mosaic-factory
+# (spawn stays consistent with the onboarding cheat-sheet + fleet ps observe).
+MOSAIC_TMUX_SOCKET=${MOSAIC_TMUX_SOCKET:-}
 MOSAIC_AGENT_RUNTIME=${MOSAIC_AGENT_RUNTIME:-pi}
+MOSAIC_AGENT_MODEL=${MOSAIC_AGENT_MODEL:-}
 MOSAIC_AGENT_WORKDIR=${MOSAIC_AGENT_WORKDIR:-$HOME}
 MOSAIC_AGENT_COMMAND=${MOSAIC_AGENT_COMMAND:-}
 MOSAIC_HEARTBEAT_RUN_DIR=${MOSAIC_HEARTBEAT_RUN_DIR:-${MOSAIC_HOME:-$HOME/.config/mosaic}/fleet/run}
@@ -19,13 +23,25 @@ if ! command -v tmux >/dev/null 2>&1; then
   exit 69
 fi
 
-if tmux -L "$MOSAIC_TMUX_SOCKET" has-session -t "=${AGENT_NAME}:0.0" 2>/dev/null; then
-  echo "Mosaic agent session already running: $AGENT_NAME on socket $MOSAIC_TMUX_SOCKET"
+# tmux wrapper: pass -L only when a socket is configured. An absent/empty socket
+# means the default tmux socket (no -L), keeping spawn == observe == cheat-sheet.
+_tmux() {
+  if [ -n "$MOSAIC_TMUX_SOCKET" ]; then
+    tmux -L "$MOSAIC_TMUX_SOCKET" "$@"
+  else
+    tmux "$@"
+  fi
+}
+
+if _tmux has-session -t "=${AGENT_NAME}:0.0" 2>/dev/null; then
+  echo "Mosaic agent session already running: $AGENT_NAME on socket ${MOSAIC_TMUX_SOCKET:-(default)}"
   exit 0
 fi
 
 if [ -z "$MOSAIC_AGENT_COMMAND" ]; then
-  MOSAIC_AGENT_COMMAND="mosaic yolo $MOSAIC_AGENT_RUNTIME"
+  # Map the roster's per-agent model_hint to `--model` so workers launch on the
+  # configured model (e.g. pi on openai-codex/gpt-5.5:high). Omitted when unset.
+  MOSAIC_AGENT_COMMAND="mosaic yolo $MOSAIC_AGENT_RUNTIME${MOSAIC_AGENT_MODEL:+ --model $MOSAIC_AGENT_MODEL}"
 fi
 
 # ── Derive a runtime-bin PATH prefix ─────────────────────────────────────────
@@ -107,13 +123,13 @@ fi
 mkdir -p "$MOSAIC_AGENT_WORKDIR"
 
 # ── Launch the tmux session (no exec — we continue to wire the heartbeat) ────
-tmux -L "$MOSAIC_TMUX_SOCKET" new-session -d -s "$AGENT_NAME" -c "$MOSAIC_AGENT_WORKDIR" \
+_tmux new-session -d -s "$AGENT_NAME" -c "$MOSAIC_AGENT_WORKDIR" \
   bash -c "$PANE_SHELL_SNIPPET"
 
 # ── Resolve the pane PID (retry briefly to let the session initialise) ────────
 PANE_PID=""
 for _retry in 1 2 3 4 5; do
-  PANE_PID=$(tmux -L "$MOSAIC_TMUX_SOCKET" list-panes \
+  PANE_PID=$(_tmux list-panes \
     -t "=${AGENT_NAME}:0.0" -F '#{pane_pid}' 2>/dev/null || true)
   [ -n "$PANE_PID" ] && break
   sleep 0.2