feat: smart update check with auto-upgrade, snooze backoff, config CLI (v0.3.9) (#62)

* feat: add bin/gstack-config CLI for reading/writing ~/.gstack/config.yaml

Simple get/set/list interface for persistent gstack configuration.
Used by update-check and upgrade skill for auto_upgrade and update_check settings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: smart update check with 12h cache, snooze backoff, config disable

- Reduce cache TTL from 24h to 12h for faster update detection
- Add exponential snooze backoff: 24h → 48h → 1 week (resets on new version)
- Add update_check: false config option to disable checks entirely
- Clear snooze file on just-upgraded
- 14 new tests covering snooze levels, expiry, corruption, and config paths

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: upgrade skill with auto-upgrade, 4-option prompt, vendored sync

- Auto-upgrade mode via config or GSTACK_AUTO_UPGRADE=1 env var
- 4-option AskUserQuestion: upgrade once, always, not now, never
- Step 4.5: sync local vendored copy after upgrading primary install
- Snooze write with escalating backoff on "Not now"
- Update preamble text in gen-skill-docs for new upgrade flow
- Regenerate all SKILL.md files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: simplify upgrade instructions, move auto-upgrade to completed

README now points to /gstack-upgrade instead of long paste commands.
Auto-upgrade TODO moved to Completed section (v0.3.8).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: bump version and changelog (v0.3.9)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

bin/gstack-config

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# gstack-config — read/write ~/.gstack/config.yaml
+#
+# Usage:
+#   gstack-config get <key>          — read a config value
+#   gstack-config set <key> <value>  — write a config value
+#   gstack-config list               — show all config
+#
+# Env overrides (for testing):
+#   GSTACK_STATE_DIR  — override ~/.gstack state directory
+set -euo pipefail
+
+STATE_DIR="${GSTACK_STATE_DIR:-$HOME/.gstack}"
+CONFIG_FILE="$STATE_DIR/config.yaml"
+
+case "${1:-}" in
+  get)
+    KEY="${2:?Usage: gstack-config get <key>}"
+    grep -E "^${KEY}:" "$CONFIG_FILE" 2>/dev/null | tail -1 | awk '{print $2}' | tr -d '[:space:]' || true
+    ;;
+  set)
+    KEY="${2:?Usage: gstack-config set <key> <value>}"
+    VALUE="${3:?Usage: gstack-config set <key> <value>}"
+    mkdir -p "$STATE_DIR"
+    if grep -qE "^${KEY}:" "$CONFIG_FILE" 2>/dev/null; then
+      sed -i '' "s/^${KEY}:.*/${KEY}: ${VALUE}/" "$CONFIG_FILE"
+    else
+      echo "${KEY}: ${VALUE}" >> "$CONFIG_FILE"
+    fi
+    ;;
+  list)
+    cat "$CONFIG_FILE" 2>/dev/null || true
+    ;;
+  *)
+    echo "Usage: gstack-config {get|set|list} [key] [value]"
+    exit 1
+    ;;
+esac