feat: add optional prefix for claude skill installs

[malikrohail]

Summary

• add ./setup --prefix <value> for Claude skill installs
• apply the prefix only to Claude-side per-skill symlink names
• avoid double-prefixing skills that already start with the chosen prefix
• add setup coverage and document the new flag in the README

Problem

Issue #284 asks for an option to prefix skill names during setup so a user-level Claude install does not drop many bare names like review, ship, and qa directly into ~/.claude/skills/.

Current main still links Claude skills without any optional prefix.

Fix

• parse --prefix / --prefix=<value> in setup
• pass the value only into link_claude_skill_dirs
• keep Codex behavior unchanged because Codex skills already use gstack-*
• preserve already-prefixed skill names such as gstack-upgrade
• document the flag in README.md

Fixes #284.

Verification

• bun test test/gen-skill-docs.test.ts
• bun test
• temp install repro:
  • create a temp .claude/skills/gstack symlink to this checkout
  • run ./setup --host claude --prefix gstack-
  • verify gstack-review exists, bare review does not, and gstack-upgrade is not double-prefixed

[malikrohail]

@garrytan verified on March 22, 2026 against current main.

I kept this scoped to the Claude install path only:

• --prefix affects Claude-side per-skill symlink names
• Codex behavior is unchanged because Codex already prefixes skill names
• there is a temp-dir end-to-end repro behind the PR body proving gstack-review is created, bare review is not, and gstack-upgrade is not double-prefixed

I did not combine this with the Codex co-author fix because they touch different review surfaces and are easier to merge separately.