u/complete: fix TOCTOU race spawning duplicate copilot sessions

Use deterministic session ID derived from cwd hash instead of
timestamp. Concurrent invocations for the same directory now
converge on the same session name, and the server rejects the
duplicate atomically.

scripts/u/complete

@@ -46,28 +46,34 @@ done
 # otherwise spawns a new one.
 find_or_create_session() {
     local target_cwd="$1"
-    local sdir state scwd id kv deadline
+    local id kv deadline
 
-    while IFS= read -r line; do
-        id=${line%%$'\t'*}
-        case "$id" in *-copilot) ;; *) continue ;; esac
-        sdir="$MOUNT/s/$id"
-        [ -d "$sdir" ] || continue
-        scwd=$(cat "$sdir/cwd" 2>/dev/null) || continue
-        [ "$scwd" = "$target_cwd" ] || continue
-        state=$(cat "$sdir/state" 2>/dev/null) || continue
-        case "$state" in failed*|error*) continue ;; esac
-        printf '%s\n' "$id"
-        return 0
-    done < "$MOUNT/s/idx"
+    # Deterministic name: one copilot session per cwd.
+    id="$(printf '%s' "$target_cwd" | cksum | awk '{print $1}')-copilot"
+
+    if [ -d "$MOUNT/s/$id" ]; then
+        local state
+        state=$(cat "$MOUNT/s/$id/state" 2>/dev/null) || true
+        case "$state" in
+            failed*|error*)
+                # Dead session — kill and recreate below.
+                printf 'kill\n' > "$MOUNT/s/$id/ctl" 2>/dev/null || true
+                sleep 0.1
+                ;;
+            *)
+                printf '%s\n' "$id"
+                return 0
+                ;;
+        esac
+    fi
 
-    id="$(date +%s%N)-copilot"
     kv="name=${id}
 cwd=${target_cwd}
 agent=copilot
 backend=${backend}
 model=${model}"
-    printf '%s\n' "$kv" > "$MOUNT/s/new" || die "failed to create copilot session"
+    # If a concurrent invocation already created it, ignore the error.
+    printf '%s\n' "$kv" > "$MOUNT/s/new" 2>/dev/null || true
 
     deadline=$(( $(date +%s) + 5 ))
     until [ -d "$MOUNT/s/$id" ] || [ "$(date +%s)" -ge "$deadline" ]; do sleep 0.05; done