feat: feature discovery flow gated by per-feature markers

Extends generateUpgradeCheck() to surface new features once per user
after a just-upgraded session. No more silent features.

Codex review caught: spawned sessions (OpenClaw, etc.) must skip the
discovery prompt entirely — they can't interactively answer. Feature
discovery now checks SPAWNED_SESSION first and is silent in those.

Discovery is per-feature, not per-upgrade. Each feature has its own
marker file at ~/.claude/skills/gstack/.feature-prompted-{name}. Once
the user has been shown a feature (accepted, shown docs, or skipped),
the marker is touched and the prompt never fires again for that
feature. Future features get their own markers.

V1 features surfaced:
- continuous-checkpoint: offer to enable checkpoint_mode=continuous
- model-overlay: inform-only note about --model flag and MODEL_OVERLAY
  line in preamble output

Max one prompt per session to avoid nagging. Fires only on JUST_UPGRADED
(not every session), plus spawned-session skip.

Changes:
- scripts/resolvers/preamble.ts: extend generateUpgradeCheck() with
  feature discovery rules, per-marker-file semantics, spawned-session
  exclusion, and max-one-per-session cap.
- All skill SKILL.md files regenerated to include the new section.
- Golden ship fixtures regenerated.

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

scripts/resolvers/preamble.ts

@@ -121,7 +121,38 @@ or invoking other gstack skills, use the \`/gstack-\` prefix (e.g., \`/gstack-qa
 of \`/qa\`, \`/gstack-ship\` instead of \`/ship\`). Disk paths are unaffected — always use
 \`${ctx.paths.skillRoot}/[skill-name]/SKILL.md\` for reading skill files.
 
-If output shows \`UPGRADE_AVAILABLE <old> <new>\`: read \`${ctx.paths.skillRoot}/gstack-upgrade/SKILL.md\` and follow the "Inline upgrade flow" (auto-upgrade if configured, otherwise AskUserQuestion with 4 options, write snooze state if declined). If \`JUST_UPGRADED <from> <to>\`: tell user "Running gstack v{to} (just updated!)" and continue.`;
+If output shows \`UPGRADE_AVAILABLE <old> <new>\`: read \`${ctx.paths.skillRoot}/gstack-upgrade/SKILL.md\` and follow the "Inline upgrade flow" (auto-upgrade if configured, otherwise AskUserQuestion with 4 options, write snooze state if declined).
+
+If output shows \`JUST_UPGRADED <from> <to>\` AND \`SPAWNED_SESSION\` is NOT set: tell
+the user "Running gstack v{to} (just updated!)" and then check for new features to
+surface. For each per-feature marker below, if the marker file is missing AND the
+feature is plausibly useful for this user, use AskUserQuestion to let them try it.
+Fire once per feature per user, NOT once per upgrade.
+
+**In spawned sessions (\`SPAWNED_SESSION\` = "true"): SKIP feature discovery entirely.**
+Just print "Running gstack v{to}" and continue. Orchestrators do not want interactive
+prompts from sub-sessions.
+
+**Feature discovery markers and prompts** (one at a time, max one per session):
+
+1. \`${ctx.paths.skillRoot}/.feature-prompted-continuous-checkpoint\` →
+   Prompt: "Continuous checkpoint auto-commits your work as you go with \`WIP:\` prefix
+   so you never lose progress to a crash. Local-only by default — doesn't push
+   anywhere unless you turn that on. Want to try it?"
+   Options: A) Enable continuous mode, B) Show me first (print the section from
+   the preamble Continuous Checkpoint Mode), C) Skip.
+   If A: run \`${ctx.paths.binDir}/gstack-config set checkpoint_mode continuous\`.
+   Always: \`touch ${ctx.paths.skillRoot}/.feature-prompted-continuous-checkpoint\`
+
+2. \`${ctx.paths.skillRoot}/.feature-prompted-model-overlay\` →
+   Inform only (no prompt): "Model overlays are active. \`MODEL_OVERLAY: {model}\`
+   shown in the preamble output tells you which behavioral patch is applied.
+   Override with \`--model\` when regenerating skills (e.g., \`bun run gen:skill-docs
+   --model gpt-5.4\`). Default is claude."
+   Always: \`touch ${ctx.paths.skillRoot}/.feature-prompted-model-overlay\`
+
+After handling JUST_UPGRADED (prompts done or skipped), continue with the skill
+workflow.`;
 }
 
 function generateLakeIntro(): string {