git clone + setup fails: skill symlinks not created when bun is not pre-installed

## Summary

When installing gstack via `git clone ... && ./setup`, the setup script fails immediately if `bun` is not installed, leaving **no skill symlinks** in `~/.claude/skills/`. The user is told to install bun manually, but even after installing bun, they must know to re-run `./setup` — which isn't obvious.

As a result, invoking any gstack sub-skill (e.g., `/office-hours`) returns `Unknown skill: office-hours` because the symlinks were never created.

## Steps to Reproduce

1. Fresh machine without `bun` installed
2. Run: `git clone https://github.com/garrytan/gstack.git ~/.claude/skills/gstack && cd ~/.claude/skills/gstack && ./setup`
3. Setup fails with: `Error: bun is required but not installed.`
4. Install bun manually: `curl -fsSL https://bun.sh/install | bash`
5. **Bun is not yet in PATH** for the current shell session
6. Re-run `./setup` — still fails unless user manually exports PATH
7. Even after fixing PATH and re-running setup, the user has already moved on and tries `/office-hours` → `Unknown skill`

## Expected Behavior

Either:
- **Option A:** `setup` auto-installs bun if missing (the script already prints the install command, could just run it)
- **Option B:** `setup` detects bun was just installed to `~/.bun/bin/` and adds it to PATH for the current invocation
- **Option C:** The README/install instructions make it explicit that bun must be installed and in PATH before running setup

## Environment

- Windows 11 (MSYS2/Git Bash)
- bun v1.3.11 (installed after initial setup failure)
- gstack latest (cloned 2026-03-25)

Start with ./setup and the README/install instructions; reproduce the clone-and-setup flow in a shell without bun, including the Windows Git Bash environment described. Compare the behavior before and after bun installation and PATH changes. Done means setup completes reliably and the expected skill symlinks exist in ~/.claude/skills/.

Written by the indexing model from the issue text.