feat: add remote slug helper and auto-gitignore for .gstack/

- getRemoteSlug() in config.ts: parses git remote origin → owner-repo format
- browse/bin/remote-slug: shell helper for SKILL.md use (BSD sed compatible)
- ensureStateDir() now appends .gstack/ to project .gitignore if not present
- setup creates ~/.gstack/projects/ global state directory
- 7 new tests: 4 gitignore behavior + 3 remote slug parsing

browse/bin/remote-slug

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# Output the remote slug (owner-repo) for the current git repo.
+# Used by SKILL.md files to derive project-specific paths in ~/.gstack/projects/.
+set -e
+URL=$(git remote get-url origin 2>/dev/null || true)
+if [ -n "$URL" ]; then
+  # Strip trailing .git if present, then extract owner/repo
+  URL="${URL%.git}"
+  # Handle both SSH (git@host:owner/repo) and HTTPS (https://host/owner/repo)
+  OWNER_REPO=$(echo "$URL" | sed -E 's#.*[:/]([^/]+)/([^/]+)$#\1-\2#')
+  echo "$OWNER_REPO"
+else
+  basename "$(git rev-parse --show-toplevel 2>/dev/null || pwd)"
+fi