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

[garagon]

Summary

./setup --prefix creates symlinks (gstack-qa → gstack/qa) but doesn't update the name: frontmatter field inside each SKILL.md. Claude Code uses name: for skill registration — so the skill registers as /qa instead of /gstack-qa, making the prefix mode non-functional.

After creating each symlink, setup now patches the name: field to match the linked name. Same fix applied to bin/gstack-relink for prefix changes after initial setup.

Root cause

link_claude_skill_dirs() reads name: from SKILL.md (line 279) and computes the correct link_name with prefix (line 282-288), but only uses it for the symlink target — the SKILL.md name: field stays unchanged. Claude Code ignores the symlink name and reads name: from frontmatter.

What changed

• setup: after ln -snf, patches name: in SKILL.md when link_name != skill_name
• bin/gstack-relink: same patch logic for prefix changes after setup

Uses portable sed (tmpfile + mv) — works on both macOS and Linux. No-op for unprefixed mode and already-prefixed skills (gstack-upgrade).

Verified

# Before (qa/SKILL.md):
name: qa          ← Claude registers as /qa

# After setup --prefix:
name: gstack-qa   ← Claude registers as /gstack-qa

Test plan

• sed patch correctly rewrites name: qa → name: gstack-qa
• Already-prefixed skills (gstack-upgrade) are unaffected (link_name == skill_name)
• Unprefixed mode (--no-prefix) does not trigger the patch
• Switching prefix modes via gstack-relink patches in both directions

Fixes #620