refactor: add PROJECTS_DIR to gstack-slug and getProjectsDir() to util.ts

gstack-slug now outputs PROJECTS_DIR (respecting GSTACK_STATE_DIR env var).
lib/util.ts gets getProjectsDir(slug?) as single source of truth for
TypeScript consumers.

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

bin/gstack-slug

@@ -5,5 +5,8 @@
 set -euo pipefail
 SLUG=$(git remote get-url origin 2>/dev/null | sed 's|.*[:/]\([^/]*/[^/]*\)\.git$|\1|;s|.*[:/]\([^/]*/[^/]*\)$|\1|' | tr '/' '-' | tr '[:upper:]' '[:lower:]')
 BRANCH=$(git rev-parse --abbrev-ref HEAD 2>/dev/null | tr '/' '-')
+STATE_DIR="${GSTACK_STATE_DIR:-$HOME/.gstack}"
+PROJECTS_DIR="${STATE_DIR}/projects"
 echo "SLUG=$SLUG"
 echo "BRANCH=$BRANCH"
+echo "PROJECTS_DIR=$PROJECTS_DIR"

lib/util.ts

@@ -15,6 +15,12 @@ import { spawnSync } from 'child_process';
 export const GSTACK_STATE_DIR = process.env.GSTACK_STATE_DIR || path.join(os.homedir(), '.gstack');
 export const GSTACK_DEV_DIR = path.join(os.homedir(), '.gstack-dev');
 
+/** Get the projects directory, optionally scoped to a specific project slug. */
+export function getProjectsDir(slug?: string): string {
+  const base = path.join(GSTACK_STATE_DIR, 'projects');
+  return slug ? path.join(base, slug) : base;
+}
+
 // --- File I/O ---
 
 /** Atomic write: write to .tmp then rename. Non-fatal on error. */