fix(setup): patch SKILL.md name: field when --prefix is used

[HMAKT99]

Summary

• when --prefix creates gstack-qa, patch the name: field in SKILL.md to match
• create a real directory (not symlink) with patched SKILL.md + symlinked subdirs
• unprefixed skills still use plain symlinks (unchanged)

Problem

./setup --prefix creates symlinks correctly:

~/.claude/skills/gstack-qa -> gstack/qa/

But the name: field inside qa/SKILL.md still says name: qa. Claude Code uses the frontmatter name: for skill registration, not the directory name.

Result: gstack-upgrade works (its SKILL.md says name: gstack-upgrade). Every other prefixed skill doesn't register because name: qa doesn't match the gstack-qa directory.

Fix

When link_name != skill_name (prefix applied):

1. Create a real directory instead of a symlink
2. Symlink subdirs (templates/, references/) back to the original so relative paths resolve
3. Write SKILL.md with sed "s/^name: $skill_name$/name: $link_name/" so the name: field matches

When link_name == skill_name (no prefix): plain symlink, unchanged.

+17/-1 in setup.

Verification

# Before fix:
head -2 ~/.claude/skills/gstack-qa/SKILL.md
  name: qa                    ← wrong

# After fix:
head -2 ~/.claude/skills/gstack-qa/SKILL.md
  name: gstack-qa             ← correct

# Subdirs still accessible through symlinks:
cat ~/.claude/skills/gstack-qa/templates/qa-report-template.md   ← works

Fixes #620.