Support stable prefixed skill names for installed skill bundles

## Problem

When GStack skills are installed into an environment that already has many skills, unprefixed names such as `qa`, `review`, `ship`, and `browse` can collide with host skills. A downstream Hermes install carries local patches that prefix generated GStack skill names, including newly added iOS skills.

## Expected behavior

GStack should support a first-class install/config mode that emits stable prefixed skill names, for example `gstack-qa`, `gstack-review`, `gstack-ios-qa`, etc., without requiring downstream hand-edits after every upstream update.

## Current downstream patch

A downstream patch rewrites generated `SKILL.md` `name:` fields across the bundle, and later repeats the same for newly introduced iOS skills. This is fragile because generated docs can be overwritten by `gen:skill-docs` or upgrades.

## Proposed direction

- Add a supported `skill_prefix` install/generation option, or
- make `bin/gstack-relink`/generation consistently preserve prefixed names for all current and future skills.