fix(uninstall): clean up per-skill directories with nested SKILL.md symlinks

[mvanhorn]

Summary

gstack-uninstall left behind per-skill directories that setup creates under ~/.claude/skills/. After gstack-uninstall --keep-state, ~30 empty directories holding broken SKILL.md symlinks into the removed gstack/ tree remained. Covers both issue reports: prefix-mode (gstack-*) from #896 and default-mode (browse/, ship/, etc.) from #1132.

Why this matters

setup (lines 399-404) creates each skill as a real directory with a nested SKILL.md symlink, regardless of prefix setting:

~/.claude/skills/autoplan/              (real dir, default mode)
└── SKILL.md -> ~/.claude/skills/gstack/autoplan/SKILL.md

~/.claude/skills/gstack-autoplan/       (real dir, --prefix mode)
└── SKILL.md -> ~/.claude/skills/gstack/autoplan/SKILL.md

The existing uninstall loop only matched top-level symlinks ([ -L "$_LINK" ]), so neither layout got cleaned up. The final rm -rf "$CLAUDE_SKILLS/gstack" then left the per-skill directories holding broken links.

Fix

Adds a second pass in the CLAUDE_SKILLS block that iterates real directories, checks whether the nested SKILL.md is a symlink pointing into gstack/, and removes the directory when it does. The legacy top-level-symlink pass is preserved so older installs still clean up.

The symlink-target check matches both default (browse/) and prefix (gstack-browse/) layouts because setup writes the same nested SKILL.md symlink pattern in both. Also safer than the earlier gstack-* glob approach, which would have over-removed any unrelated tool whose top-level directory happened to start with gstack-.

Scoped to ~/.claude/skills. The codex/factory/kiro paths use a different install topology and are untouched.

Test

Added test('current setup layout...') in test/uninstall.test.ts covering the real-dir-plus-nested-symlink layout. Runs --force, asserts per-skill directories are gone, confirms a sibling with no SKILL.md survives.

Local verification on a fake HOME:

BEFORE:  ~/.claude/skills/{autoplan, ship, gstack, not-gstack}
AFTER:   ~/.claude/skills/{not-gstack}    (autoplan and ship removed; not-gstack preserved)

bun test test/uninstall.test.ts: 8/8 pass.

Fixes #896, fixes #1132

[mvanhorn]

Thanks for digging through this @dgrant. The symlink-aware uninstall covers both #896 and your #1132 case, so the test/uninstall.test.ts assertions should match what you were hitting locally.

[capy-ai]

Thank you for this fix! The bug it addresses (#896, #1132) has since been fixed on main and the issue is closed, so I'm closing this PR as done. Re-verified against current main (v1.91.12.0). If you think part of your change is still missing, reply and we'll reopen. Really appreciate the contribution.