feat: lifecycle map + proactive preference system

Root gstack description now includes a developer workflow guide mapping
12 stages to skills. Preamble reads proactive preference via gstack-config.
Users can opt out with "stop suggesting things" and re-enable with
"be proactive again" — natural language toggle, no CLI needed.

SKILL.md.tmpl

@@ -7,6 +7,32 @@ description: |
   responsive layouts, test forms and uploads, handle dialogs, and assert element states.
   ~100ms per command. Use when you need to test a feature, verify a deployment, dogfood a
   user flow, or file a bug with evidence.
+
+  gstack also includes development workflow skills. When you notice the user is at
+  these stages, suggest the appropriate skill:
+  - Brainstorming a new idea → suggest /office-hours
+  - Reviewing a plan (strategy) → suggest /plan-ceo-review
+  - Reviewing a plan (architecture) → suggest /plan-eng-review
+  - Reviewing a plan (design) → suggest /plan-design-review
+  - Creating a design system → suggest /design-consultation
+  - Debugging errors → suggest /debug
+  - Testing the app → suggest /qa
+  - Code review before merge → suggest /review
+  - Visual design audit → suggest /design-review
+  - Ready to deploy / create PR → suggest /ship
+  - Post-ship doc updates → suggest /document-release
+  - Weekly retrospective → suggest /retro
+
+  If the user pushes back on skill suggestions ("stop suggesting things",
+  "I don't need suggestions", "too aggressive"):
+  1. Stop suggesting for the rest of this session
+  2. Run: gstack-config set proactive false
+  3. Say: "Got it — I'll stop suggesting skills. Just tell me to be proactive
+     again if you change your mind."
+
+  If the user says "be proactive again" or "turn on suggestions":
+  1. Run: gstack-config set proactive true
+  2. Say: "Proactive suggestions are back on."
 allowed-tools:
   - Bash
   - Read