feat: extend gstack-gbrain-detect with mcp_mode + artifacts_remote

Adds two new fields to detect's JSON output:

- gbrain_mcp_mode: local-stdio | remote-http | none
  Resolved via 3-tier fallback (codex Finding D3): claude mcp get --json
  → claude mcp list text-grep → ~/.claude.json jq read. If Anthropic moves
  the file format, the first two tiers absorb it.

- gstack_artifacts_remote: HTTPS URL from ~/.gstack-artifacts-remote.txt
  Falls back to ~/.gstack-brain-remote.txt during the v1.27.0.0 migration
  window so detect doesn't return empty between upgrade and migration.

Existing detect tests still pass (15/15). New 19 tests cover every fallback
tier independently, plus a schema regression for /sync-gbrain compat.

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

bin/gstack-gbrain-detect

@@ -11,8 +11,10 @@
 #     "gbrain_config_exists": true|false,
 #     "gbrain_engine": "pglite"|"postgres" | null,
 #     "gbrain_doctor_ok": true|false,
+#     "gbrain_mcp_mode": "local-stdio"|"remote-http"|"none",
 #     "gstack_brain_sync_mode": "off"|"artifacts-only"|"full",
-#     "gstack_brain_git": true|false
+#     "gstack_brain_git": true|false,
+#     "gstack_artifacts_remote": "https://..." | ""
 #   }
 #
 # The /setup-gbrain skill reads this once at startup to decide which path
@@ -92,6 +94,76 @@ if [ -d "$STATE_DIR/.git" ]; then
   gstack_brain_git=true
 fi
 
+# --- gbrain_mcp_mode: local-stdio | remote-http | none ---
+# Defense-in-depth fallback chain (intentional ordering, do not reorder):
+#   1. `claude mcp get gbrain --json` — public CLI surface, structured output
+#   2. `claude mcp list` text-grep    — older claude versions without --json
+#   3. `~/.claude.json` jq read       — last resort if `claude` isn't on PATH
+# Fallback chain logged because if Anthropic moves the file or renames keys,
+# the third tier breaks silently; the first two tiers should catch it.
+gbrain_mcp_mode="none"
+if command -v claude >/dev/null 2>&1; then
+  # Tier 1: claude mcp get --json
+  if mcp_get_json=$(claude mcp get gbrain --json 2>/dev/null); then
+    if echo "$mcp_get_json" | jq -e '.' >/dev/null 2>&1; then
+      mtype=$(echo "$mcp_get_json" | jq -r '.type // .transport // empty' 2>/dev/null)
+      mcommand=$(echo "$mcp_get_json" | jq -r '.command // empty' 2>/dev/null)
+      murl=$(echo "$mcp_get_json" | jq -r '.url // empty' 2>/dev/null)
+      case "$mtype" in
+        http|sse) gbrain_mcp_mode="remote-http" ;;
+        stdio)    gbrain_mcp_mode="local-stdio" ;;
+        *)
+          # Newer claude versions may emit just url + command; infer.
+          if [ -n "$murl" ]; then gbrain_mcp_mode="remote-http"
+          elif [ -n "$mcommand" ]; then gbrain_mcp_mode="local-stdio"
+          fi
+          ;;
+      esac
+    fi
+  fi
+  # Tier 2: claude mcp list text-grep (only if Tier 1 didn't resolve)
+  if [ "$gbrain_mcp_mode" = "none" ]; then
+    if mcp_list=$(claude mcp list 2>/dev/null); then
+      gbrain_line=$(echo "$mcp_list" | grep -E '^gbrain:' || true)
+      if [ -n "$gbrain_line" ]; then
+        if echo "$gbrain_line" | grep -q 'http\|HTTP'; then
+          gbrain_mcp_mode="remote-http"
+        else
+          gbrain_mcp_mode="local-stdio"
+        fi
+      fi
+    fi
+  fi
+fi
+# Tier 3: ~/.claude.json jq read (only if claude binary or earlier tiers failed)
+if [ "$gbrain_mcp_mode" = "none" ]; then
+  if [ -f "$HOME/.claude.json" ]; then
+    # Look for a gbrain MCP server entry. Type field disambiguates http vs stdio.
+    mtype=$(jq -r '.mcpServers.gbrain.type // .mcpServers.gbrain.transport // empty' "$HOME/.claude.json" 2>/dev/null)
+    murl=$(jq -r '.mcpServers.gbrain.url // empty' "$HOME/.claude.json" 2>/dev/null)
+    mcommand=$(jq -r '.mcpServers.gbrain.command // empty' "$HOME/.claude.json" 2>/dev/null)
+    case "$mtype" in
+      url|http|sse) gbrain_mcp_mode="remote-http" ;;
+      stdio)        gbrain_mcp_mode="local-stdio" ;;
+      *)
+        if [ -n "$murl" ]; then gbrain_mcp_mode="remote-http"
+        elif [ -n "$mcommand" ]; then gbrain_mcp_mode="local-stdio"
+        fi
+        ;;
+    esac
+  fi
+fi
+
+# --- artifacts remote URL (post-rename) with brain-* fallback during the
+#     migration window (gstack-upgrade migration runs the rename). ---
+gstack_artifacts_remote=""
+if [ -f "$HOME/.gstack-artifacts-remote.txt" ]; then
+  gstack_artifacts_remote=$(head -1 "$HOME/.gstack-artifacts-remote.txt" 2>/dev/null | tr -d '[:space:]' || true)
+elif [ -f "$HOME/.gstack-brain-remote.txt" ]; then
+  # Pre-migration fallback. Migration v1.27.0.0 will mv this to the new path.
+  gstack_artifacts_remote=$(head -1 "$HOME/.gstack-brain-remote.txt" 2>/dev/null | tr -d '[:space:]' || true)
+fi
+
 # Emit single-object JSON.
 jq -n \
   --argjson on_path "$gbrain_on_path" \
@@ -99,14 +171,18 @@ jq -n \
   --argjson config_exists "$gbrain_config_exists" \
   --argjson engine "$gbrain_engine" \
   --argjson doctor_ok "$gbrain_doctor_ok" \
+  --arg mcp_mode "$gbrain_mcp_mode" \
   --arg sync_mode "$gstack_brain_sync_mode" \
   --argjson brain_git "$gstack_brain_git" \
+  --arg artifacts_remote "$gstack_artifacts_remote" \
   '{
     gbrain_on_path: $on_path,
     gbrain_version: $version,
     gbrain_config_exists: $config_exists,
     gbrain_engine: $engine,
     gbrain_doctor_ok: $doctor_ok,
+    gbrain_mcp_mode: $mcp_mode,
     gstack_brain_sync_mode: $sync_mode,
-    gstack_brain_git: $brain_git
+    gstack_brain_git: $brain_git,
+    gstack_artifacts_remote: $artifacts_remote
   }'