fix(uninstall): remove real-directory skill installs, gated on provenance

On Windows, setup installs skills as REAL directory copies (cp -R via
_link_or_copy). gstack-uninstall's per-skill loop filtered on [ -L ], so
every copy was skipped: --force exited 0 and printed 'gstack uninstalled.'
while leaving ~52 gstack-* directories plus _gstack-command/ behind in
~/.claude/skills. The same filter also missed the standard Unix shape (real
dir + symlinked SKILL.md), which was left as a dangling-symlink husk.

Fix: the loop now handles all three install shapes. Symlink entries keep
the existing readlink check. Real dirs with a SYMLINKED SKILL.md are removed
when the link points into gstack (same semantics as setup's cleanup
helpers). Real dirs with a REAL-FILE SKILL.md — the Windows copy shape — are
removed ONLY when both provenance gates pass (F8): (a) the directory name is
in gstack's skill inventory (source dir names, frontmatter names, gstack-
prefixed variants, and the alias dirs), and (b) the SKILL.md carries the
existing generated banner '<!-- AUTO-GENERATED from' (ENG-OV10: every
pre-v1.67 copy already carries it; a NEW marker would refuse to delete
legitimate old installs, recreating the bug). Anything failing a gate is
listed to stderr and never deleted — a user's own skill that happens to
share a name with a gstack skill survives.

Tests: a fake-tree fixture covers removed/kept/listed for every shape
(including the F8 name-collision row), and a census test asserts every
installable skill's generated SKILL.md carries the banner so the gate can't
strand a bannerless skill. Registered in the Windows-safe curated list —
the copy shape is exactly what windows-latest exercises.

Fixes #2563

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/test-free-shards.ts

@@ -265,6 +265,16 @@ const KNOWN_WINDOWS_SAFE: Array<{ file: string; reason: string }> = [
     // platform it bites.
     reason: 'bin/ hits are fixture path segments; spawns bash explicitly — the IS_WINDOWS=1 refresh path must run on windows-latest',
   },
+  {
+    file: 'test/uninstall-windows-copies.test.ts',
+    // Trips the "spawns bin/ shebang script" pattern via the
+    // path.join(ROOT, 'bin', 'gstack-uninstall') constant, but the script is
+    // always spawned through spawnSync('bash', [UNINSTALL, ...]). This file
+    // carries the #2563 Windows real-dir-copy uninstall coverage — the bug
+    // ONLY reproduces on the copy install shape windows-latest exercises.
+    // The symlink-shape describe block self-skips on win32.
+    reason: 'bin/ hit is a bash-spawned script path; #2563 real-dir uninstall coverage must run on windows-latest',
+  },
   {
     file: 'browse/test/file-permissions.test.ts',
     // Trips the POSIX-mode-bitmask pattern, but every `mode & 0o777` assertion