feat: bin/gstack-next-version util + workspace_root config key

Host-aware (GitHub + GitLab + unknown) VERSION allocator. Queries the open
PR queue, fetches each PR's VERSION at head, scans configurable Conductor
sibling worktrees for WIP work, and picks the next free slot at the
requested bump level.

Pure reader, never writes files. /ship consumes the JSON and decides.

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

bin/gstack-config

@@ -64,6 +64,13 @@ CONFIG_HEADER='# gstack configuration — edit freely, changes take effect on ne
 # gstack_contributor: false # true = file field reports when gstack misbehaves
 # skip_eng_review: false    # true = skip eng review gate in /ship (not recommended)
 #
+# ─── Workspace-aware ship ────────────────────────────────────────────
+# workspace_root: $HOME/conductor/workspaces  # Where /ship looks for sibling
+#                           # Conductor worktrees when picking a VERSION slot.
+#                           # Set to "null" to disable sibling scanning entirely.
+#                           # Non-Conductor users can point this at any directory
+#                           # that holds parallel worktrees of the same repo.
+#
 '
 
 # DEFAULTS table — canonical default values for known keys.
@@ -82,6 +89,7 @@ lookup_default() {
     codex_reviews) echo "enabled" ;;
     gstack_contributor) echo "false" ;;
     skip_eng_review) echo "false" ;;
+    workspace_root) echo "$HOME/conductor/workspaces" ;;
     cross_project_learnings) echo "" ;; # intentionally empty → unset triggers first-time prompt
     *) echo "" ;;
   esac
@@ -142,7 +150,7 @@ case "${1:-}" in
     echo "# ─── Active values (including defaults for unset keys) ───"
     for KEY in proactive routing_declined telemetry auto_upgrade update_check \
                skill_prefix checkpoint_mode checkpoint_push codex_reviews \
-               gstack_contributor skip_eng_review; do
+               gstack_contributor skip_eng_review workspace_root; do
       VALUE=$(grep -E "^${KEY}:" "$CONFIG_FILE" 2>/dev/null | tail -1 | awk '{print $2}' | tr -d '[:space:]' || true)
       SOURCE="default"
       if [ -n "$VALUE" ]; then
@@ -157,7 +165,7 @@ case "${1:-}" in
     echo "# gstack-config defaults"
     for KEY in proactive routing_declined telemetry auto_upgrade update_check \
                skill_prefix checkpoint_mode checkpoint_push codex_reviews \
-               gstack_contributor skip_eng_review; do
+               gstack_contributor skip_eng_review workspace_root; do
       printf '  %-24s %s\n' "$KEY:" "$(lookup_default "$KEY")"
     done
     ;;