feat: one-time CLAUDE.md routing injection prompt

Add a preamble section that checks if the project's CLAUDE.md has
skill routing rules. If not (and user hasn't declined), asks once
via AskUserQuestion to inject a "## Skill routing" section.

Root cause: skill descriptions in system-reminder metadata are too
weak to reliably trigger proactive Skill tool invocation. CLAUDE.md
project instructions carry higher weight in Claude's decision making.

- Preamble bash checks for "## Skill routing" in CLAUDE.md
- Stores decline in gstack-config (routing_declined=true)
- Only asks once per project (HAS_ROUTING check + config check)

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

scripts/resolvers/preamble.ts

@@ -74,6 +74,14 @@ if [ -f "$_LEARN_FILE" ]; then
 else
   echo "LEARNINGS: 0"
 fi
+# Check if CLAUDE.md has routing rules
+_HAS_ROUTING="no"
+if [ -f CLAUDE.md ] && grep -q "## Skill routing" CLAUDE.md 2>/dev/null; then
+  _HAS_ROUTING="yes"
+fi
+_ROUTING_DECLINED=$(${ctx.paths.binDir}/gstack-config get routing_declined 2>/dev/null || echo "false")
+echo "HAS_ROUTING: $_HAS_ROUTING"
+echo "ROUTING_DECLINED: $_ROUTING_DECLINED"
 \`\`\``;
 }
 
@@ -164,6 +172,51 @@ touch ~/.gstack/.proactive-prompted
 This only happens once. If \`PROACTIVE_PROMPTED\` is \`yes\`, skip this entirely.`;
 }
 
+function generateRoutingInjection(ctx: TemplateContext): string {
+  return `If \`HAS_ROUTING\` is \`no\` AND \`ROUTING_DECLINED\` is \`false\` AND \`PROACTIVE_PROMPTED\` is \`yes\`:
+Check if a CLAUDE.md file exists in the project root. If it does not exist, create it.
+
+Use AskUserQuestion:
+
+> gstack works best when your project's CLAUDE.md includes skill routing rules.
+> This tells Claude to use specialized workflows (like /ship, /investigate, /qa)
+> instead of answering directly. It's a one-time addition, about 15 lines.
+
+Options:
+- A) Add routing rules to CLAUDE.md (recommended)
+- B) No thanks, I'll invoke skills manually
+
+If A: Append this section to the end of CLAUDE.md:
+
+\`\`\`markdown
+
+## Skill routing
+
+When the user's request matches an available skill, ALWAYS invoke it using the Skill
+tool as your FIRST action. Do NOT answer directly, do NOT use other tools first.
+The skill has specialized workflows that produce better results than ad-hoc answers.
+
+Key routing rules:
+- Product ideas, "is this worth building", brainstorming → invoke office-hours
+- Bugs, errors, "why is this broken", 500 errors → invoke investigate
+- Ship, deploy, push, create PR → invoke ship
+- QA, test the site, find bugs → invoke qa
+- Code review, check my diff → invoke review
+- Update docs after shipping → invoke document-release
+- Weekly retro → invoke retro
+- Design system, brand → invoke design-consultation
+- Visual audit, design polish → invoke design-review
+- Architecture review → invoke plan-eng-review
+\`\`\`
+
+Then commit the change: \`git add CLAUDE.md && git commit -m "chore: add gstack skill routing rules to CLAUDE.md"\`
+
+If B: run \`${ctx.paths.binDir}/gstack-config set routing_declined true\`
+Say "No problem. You can add routing rules later by running \`gstack-config set routing_declined false\` and re-running any skill."
+
+This only happens once per project. If \`HAS_ROUTING\` is \`yes\` or \`ROUTING_DECLINED\` is \`true\`, skip this entirely.`;
+}
+
 function generateAskUserFormat(_ctx: TemplateContext): string {
   return `## AskUserQuestion Format
 
@@ -520,6 +573,7 @@ export function generatePreamble(ctx: TemplateContext): string {
     generateLakeIntro(),
     generateTelemetryPrompt(ctx),
     generateProactivePrompt(ctx),
+    generateRoutingInjection(ctx),
     generateVoiceDirective(tier),
     ...(tier >= 2 ? [generateAskUserFormat(ctx), generateCompletenessSection()] : []),
     ...(tier >= 3 ? [generateRepoModeSection(), generateSearchBeforeBuildingSection(ctx)] : []),