fix: pre-landing review auto-fixes (5 correctness + observability)

/ship Step 9 review surfaced 9 INFORMATIONAL findings on the new helper +
migration. Five auto-fixed with no behavior regression (26/26 tests pass):

bin/gstack-gbrain-source-wireup:
- Version compare: put floor "0.18.0" first in `sort -V` stdin so equal-or-
  greater $v always sorts to position 2. Stable across sort implementations.
- _worktree_add_detached: drop `2>/dev/null` on the `worktree add`, surface
  git's stderr through `prefix` so users see WHY adds fail (disk, perms).
- ensure_worktree: same observability fix on the `git checkout --detach` path
  during HEAD-advance, so users see the actual git error before recovery.
- do_probe: replace `[ -d X ] || [ -f X ] && set=present` (precedence trap —
  the `&&` short-circuits when the dir branch fails) with explicit if-block.
- do_probe: capture `check_source_state`'s return code explicitly via
  `set +e; ...; rc=$?; set -e`. `$?` after an `if`/`elif` chain is fragile
  under set -e and may not reach the elif under some shell versions.
- do_wireup: same explicit return-code capture for `ensure_worktree`. The
  prior `ensure_worktree || { if [ $? = 2 ]; ...` pattern relied on `$?`
  reflecting the function's return after `||`, which is implementation-defined.

gstack-upgrade/migrations/v1.15.1.0.sh:
- Trim whitespace from `gstack-config get gbrain_sync_mode` output via
  `tr -d '[:space:]'`. Trailing newlines would mis-classify "off\n" as a
  non-empty non-off mode and incorrectly invoke the helper.

Skipped findings (cosmetic / out of scope):
- `python3 -c` reads `~/.gbrain/config.json` via `expanduser` instead of
  the helper's `$GBRAIN_CONFIG` variable (cosmetic; HONORS HOME override).
- Long sync-failure error message could truncate to last N lines (cosmetic
  log readability).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

bin/gstack-gbrain-source-wireup

@@ -127,8 +127,9 @@ gbrain_version_ok() {
   local v
   v=$(gbrain --version 2>/dev/null | awk '{print $2}')
   [ -z "$v" ] && return 1
-  # 0.18.0 minimum (gbrain sources shipped here)
-  [ "$(printf '%s\n0.18.0\n' "$v" | sort -V | head -1)" = "0.18.0" ]
+  # 0.18.0 minimum (gbrain sources shipped here). Put the floor first in stdin
+  # so equal or greater $v sorts to position 2 — head -1 == "0.18.0" iff $v >= floor.
+  [ "$(printf '0.18.0\n%s\n' "$v" | sort -V | head -1)" = "0.18.0" ]
 }
 
 # ---- worktree management ----
@@ -140,7 +141,9 @@ _worktree_add_detached() {
   local sha
   sha=$(git -C "$GSTACK_HOME" rev-parse HEAD 2>/dev/null) || return 1
   git -C "$GSTACK_HOME" worktree prune 2>/dev/null || true
-  git -C "$GSTACK_HOME" worktree add --detach "$WORKTREE" "$sha" 2>/dev/null
+  # Surface git errors via prefix so users see WHY the add failed (disk, perms, etc).
+  git -C "$GSTACK_HOME" worktree add --detach "$WORKTREE" "$sha" 2>&1 | prefix
+  return "${PIPESTATUS[0]}"
 }
 
 ensure_worktree() {
@@ -152,7 +155,9 @@ ensure_worktree() {
     if [ "$NO_PULL" = "0" ]; then
       local sha
       sha=$(git -C "$GSTACK_HOME" rev-parse HEAD 2>/dev/null) || return 1
-      ( cd "$WORKTREE" && git checkout --detach "$sha" 2>/dev/null ) || {
+      # Surface checkout errors via prefix so users see WHY the advance failed
+      # (uncommitted changes in the detached worktree, ref ambiguity, etc).
+      ( cd "$WORKTREE" && git checkout --detach "$sha" 2>&1 | prefix; exit "${PIPESTATUS[0]}" ) || {
         warn "worktree at $WORKTREE could not advance to $sha; resetting via remove + re-add"
         git -C "$GSTACK_HOME" worktree remove --force "$WORKTREE" 2>/dev/null || rm -rf "$WORKTREE"
         _worktree_add_detached || return 1
@@ -185,14 +190,23 @@ check_source_state() {
 do_probe() {
   local id worktree_status="absent" gbrain_status="missing" source_status="absent"
   id=$(derive_source_id 2>/dev/null) || id="(unknown)"
-  [ -d "$WORKTREE/.git" ] || [ -f "$WORKTREE/.git" ] && worktree_status="present"
+  # Use explicit if-block so [ -d ] || [ -f ] doesn't get short-circuited by &&
+  # precedence (the `||` and `&&` chain has trap behavior in bash test syntax).
+  if [ -d "$WORKTREE/.git" ] || [ -f "$WORKTREE/.git" ]; then
+    worktree_status="present"
+  fi
   if gbrain_version_ok; then
     gbrain_status="ok ($(gbrain --version 2>/dev/null | awk '{print $2}'))"
-    if check_source_state "$id"; then
-      source_status="registered ($WORKTREE)"
-    elif [ $? = 1 ]; then
-      source_status="registered (different path)"
-    fi
+    # Capture check_source_state's return code explicitly. Relying on $? after
+    # an `if`-elif chain is fragile under set -e and undefined under some shells.
+    set +e
+    check_source_state "$id"
+    local css_rc=$?
+    set -e
+    case "$css_rc" in
+      0) source_status="registered ($WORKTREE)" ;;
+      1) source_status="registered (different path)" ;;
+    esac
   fi
   echo "source_id=$id"
   echo "worktree=$WORKTREE"
@@ -213,14 +227,22 @@ do_wireup() {
     exit 0
   fi
 
-  ensure_worktree || {
-    if [ $? = 2 ]; then
+  # Capture ensure_worktree's return code explicitly. `$?` after `||` reflects
+  # the LAST command in the function under set -e, which is unreliable when the
+  # function has multiple internal exit paths.
+  set +e
+  ensure_worktree
+  ew_rc=$?
+  set -e
+  case "$ew_rc" in
+    0) : ;;  # success
+    2)
       [ "$STRICT" = "1" ] && die "no $GSTACK_HOME/.git; run /setup-gbrain Step 7 (gstack-brain-init) first" 2
       warn "no $GSTACK_HOME/.git; skipping (benign skip)"
       exit 0
-    fi
-    die "git worktree creation failed at $WORKTREE" 1
-  }
+      ;;
+    *) die "git worktree creation failed at $WORKTREE" 1 ;;
+  esac
 
   # Source registration: probe state, then act.
   set +e

gstack-upgrade/migrations/v1.15.1.0.sh

@@ -29,7 +29,9 @@ WIREUP_BIN="${BIN_DIR}/gstack-gbrain-source-wireup"
 # Skip if user never opted into brain-sync.
 SYNC_MODE=""
 if [ -x "$CONFIG_BIN" ]; then
-  SYNC_MODE=$("$CONFIG_BIN" get gbrain_sync_mode 2>/dev/null || echo "")
+  # Trim whitespace defensively: gstack-config can emit trailing newlines,
+  # which would mis-classify "off\n" as a non-empty non-off mode.
+  SYNC_MODE=$("$CONFIG_BIN" get gbrain_sync_mode 2>/dev/null | tr -d '[:space:]' || echo "")
 fi
 if [ "$SYNC_MODE" = "off" ] || [ -z "$SYNC_MODE" ]; then
   exit 0