feat: add skills prefix

[emersonding]

Since it's many skills and often installing to .claude user dir, an option to enable skill prefix when running setup will be helpful, e.g. gstack-office-hour.

[emersonding]

Currently make them project-level skills, work as well.

[malikrohail]

Tested on March 22, 2026 against current main. This is still unimplemented: Claude installs still link bare skill names directly into ~/.claude/skills/.

I opened PR #336 with a focused implementation:

• add ./setup --prefix <value> / --prefix=<value>
• apply it only to Claude-side per-skill symlink names
• leave Codex installs unchanged because they already use gstack-*
• avoid double-prefixing gstack-upgrade
• add setup tests and a README note

PR: #336