feat(ship): plan-tune discoverability nudge after first successful ship

Plan-tune cathedral T15 (the ship-side surface; the setup-side surface
shipped in T8 with explicit hook-install consent UX). Adds Step 21 to
ship/SKILL.md.tmpl: after Step 20 (persist metrics) succeeds, surface
/plan-tune once per machine via a marker-gated single-line nudge.

Behavior:
- If ~/.gstack/.plan-tune-nudge-shown exists → no-op.
- If question_tuning is already true → no-op (user already on board).
- Otherwise: print one nudge line, touch marker.

The nudge mentions both the observational substrate AND the hook-installed
auto-decide enforcement so users know what they get when they opt in.
Non-blocking — never asks a question, doesn't gate ship completion.

To re-show: rm ~/.gstack/.plan-tune-nudge-shown before next ship.

Setup-side discoverability shipped in T8 via the hook install prompt
(explicit consent + diff preview + backup). Together these two surfaces
cover first-install AND first-ship moments — the user discovers plan-tune
organically rather than needing to know /plan-tune exists.

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

Author: garrytan

ship/SKILL.md

@@ -3086,6 +3086,29 @@ This step is automatic — never skip it, never ask for confirmation.
 
 ---
 
+## Step 21: Plan-tune discoverability nudge (first-successful-ship only)
+
+Plan-tune cathedral T15. After a successful ship, surface /plan-tune once
+per machine. Single line, non-blocking, marker-gated so it never re-fires.
+
+```bash
+_NUDGE_MARKER="$HOME/.gstack/.plan-tune-nudge-shown"
+_QT=$(~/.claude/skills/gstack/bin/gstack-config get question_tuning 2>/dev/null || echo "false")
+if [ ! -f "$_NUDGE_MARKER" ] && [ "$_QT" = "false" ]; then
+  echo ""
+  echo "gstack can learn from your AskUserQuestion answers. Run /plan-tune to opt in"
+  echo "— it captures which prompts you find valuable vs noisy and (with hooks installed)"
+  echo "auto-decides your never-ask preferences."
+  touch "$_NUDGE_MARKER"
+fi
+```
+
+If the marker exists, OR question_tuning is already on, the nudge is a
+no-op. The marker guarantees at-most-once per machine. To re-enable:
+`rm ~/.gstack/.plan-tune-nudge-shown` before next ship.
+
+---
+
 ## Important Rules
 
 - **Never skip tests.** If tests fail, stop.

ship/SKILL.md.tmpl

@@ -975,6 +975,29 @@ This step is automatic — never skip it, never ask for confirmation.
 
 ---
 
+## Step 21: Plan-tune discoverability nudge (first-successful-ship only)
+
+Plan-tune cathedral T15. After a successful ship, surface /plan-tune once
+per machine. Single line, non-blocking, marker-gated so it never re-fires.
+
+```bash
+_NUDGE_MARKER="$HOME/.gstack/.plan-tune-nudge-shown"
+_QT=$(~/.claude/skills/gstack/bin/gstack-config get question_tuning 2>/dev/null || echo "false")
+if [ ! -f "$_NUDGE_MARKER" ] && [ "$_QT" = "false" ]; then
+  echo ""
+  echo "gstack can learn from your AskUserQuestion answers. Run /plan-tune to opt in"
+  echo "— it captures which prompts you find valuable vs noisy and (with hooks installed)"
+  echo "auto-decides your never-ask preferences."
+  touch "$_NUDGE_MARKER"
+fi
+```
+
+If the marker exists, OR question_tuning is already on, the nudge is a
+no-op. The marker guarantees at-most-once per machine. To re-enable:
+`rm ~/.gstack/.plan-tune-nudge-shown` before next ship.
+
+---
+
 ## Important Rules
 
 - **Never skip tests.** If tests fail, stop.