feat: add vendoring deprecation detection to skill preamble

Detects vendored gstack in CWD (.claude/skills/gstack/ that's not a
symlink and has VERSION or .git). Outputs VENDORED_GSTACK: yes/no.
Adds generateVendoringDeprecation() section that offers one-time
migration to team mode via AskUserQuestion.

Part of team-install-mode feature (credit: Jared Friedman).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/resolvers/preamble.ts

@@ -89,6 +89,14 @@ fi
 _ROUTING_DECLINED=$(${ctx.paths.binDir}/gstack-config get routing_declined 2>/dev/null || echo "false")
 echo "HAS_ROUTING: $_HAS_ROUTING"
 echo "ROUTING_DECLINED: $_ROUTING_DECLINED"
+# Vendoring deprecation: detect if CWD has a vendored gstack copy
+_VENDORED="no"
+if [ -d ".claude/skills/gstack" ] && [ ! -L ".claude/skills/gstack" ]; then
+  if [ -f ".claude/skills/gstack/VERSION" ] || [ -d ".claude/skills/gstack/.git" ]; then
+    _VENDORED="yes"
+  fi
+fi
+echo "VENDORED_GSTACK: $_VENDORED"
 \`\`\``;
 }
 
@@ -226,6 +234,40 @@ Say "No problem. You can add routing rules later by running \`gstack-config set
 This only happens once per project. If \`HAS_ROUTING\` is \`yes\` or \`ROUTING_DECLINED\` is \`true\`, skip this entirely.`;
 }
 
+function generateVendoringDeprecation(ctx: TemplateContext): string {
+  return `If \`VENDORED_GSTACK\` is \`yes\`: This project has a vendored copy of gstack at
+\`.claude/skills/gstack/\`. Vendoring is deprecated. We will not keep vendored copies
+up to date, so this project's gstack will fall behind.
+
+Use AskUserQuestion (one-time per project, check for \`~/.gstack/.vendoring-warned-$SLUG\` marker):
+
+> This project has gstack vendored in \`.claude/skills/gstack/\`. Vendoring is deprecated.
+> We won't keep this copy up to date, so you'll fall behind on new features and fixes.
+>
+> Want to migrate to team mode? It takes about 30 seconds.
+
+Options:
+- A) Yes, migrate to team mode now
+- B) No, I'll handle it myself
+
+If A:
+1. Run \`git rm -r .claude/skills/gstack/\`
+2. Run \`echo '.claude/skills/gstack/' >> .gitignore\`
+3. Run \`${ctx.paths.binDir}/gstack-team-init required\` (or \`optional\`)
+4. Run \`git add .claude/ .gitignore CLAUDE.md && git commit -m "chore: migrate gstack from vendored to team mode"\`
+5. Tell the user: "Done. Each developer now runs: \`cd ~/.claude/skills/gstack && ./setup --team\`"
+
+If B: say "OK, you're on your own to keep the vendored copy up to date."
+
+Always run (regardless of choice):
+\`\`\`bash
+eval "$(${ctx.paths.binDir}/gstack-slug 2>/dev/null)" 2>/dev/null || true
+touch ~/.gstack/.vendoring-warned-\${SLUG:-unknown}
+\`\`\`
+
+This only happens once per project. If the marker file exists, skip entirely.`;
+}
+
 function generateAskUserFormat(_ctx: TemplateContext): string {
   return `## AskUserQuestion Format
 
@@ -675,6 +717,7 @@ export function generatePreamble(ctx: TemplateContext): string {
     generateTelemetryPrompt(ctx),
     generateProactivePrompt(ctx),
     generateRoutingInjection(ctx),
+    generateVendoringDeprecation(ctx),
     generateVoiceDirective(tier),
     ...(tier >= 2 ? [generateContextRecovery(ctx), generateAskUserFormat(ctx), generateCompletenessSection()] : []),
     ...(tier >= 3 ? [generateRepoModeSection(), generateSearchBeforeBuildingSection(ctx)] : []),