fix(relink): don't clobber foreign skills that share a gstack name

[smblight]

Problem

Installing gstack with --prefix (or any later ./setup run) silently deletes a user's own flat-named skills when they share a name with a gstack skill. Concretely: a personal /qa or /review skill symlinked into ~/.agents/skills/ gets removed.

It's not a one-time hit — setup calls gstack-relink as a self-healing step on every run, so each upgrade re-deletes them.

Root cause

bin/gstack-relink → _cleanup_skill_entry() removed any flat-named entry matching a gstack skill name, with no check on where the symlink points:

if [ -L "$entry" ]; then
  rm -f "$entry"            # deletes foreign symlinks too

In prefix mode the deletion isn't even necessary — gstack installs under gstack-* names — so the removed flat entry was pure collateral.

Fix

Scope the cleanup to gstack-owned entries only: remove a flat entry just when its symlink (or its dir's SKILL.md symlink) resolves into the gstack install dir. This mirrors the provenance check setup's cleanup_old_claude_symlinks() already uses, and also handles installs not literally named gstack/ via an $INSTALL_DIR-prefix match.

Result: foreign skills sharing a gstack name are preserved; genuinely stale flat-mode gstack entries are still cleaned.

Tests

Two regression tests added to test/relink.test.ts:

• prefix mode preserves foreign flat skills sharing a gstack name — covers both the directory-symlink shape and the real-dir-with-foreign-SKILL.md-symlink shape.
• prefix mode still removes gstack-owned stale flat entries — guards against over-correcting.

Verified the first test fails on main (foreign /qa deleted → ENOENT) and passes with the fix. Full suite: 26 pass, 0 fail.

bun test test/relink.test.ts

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