fix(pipeline): a slug used in two places is refused even when one is the wing's own (#208)

FindSlug returned the first hit, so a wing's own page of a name masked another
page of the same name (the retired dirs are searched before the wings, and wings
alphabetically). FindSlugs returns every match and callers refuse any that is
not their own. Found by the legacy-move planner's own test.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ingestion/internal/pipeline/wing_layout_test.go

@@ -140,3 +140,17 @@ func listAll(t *testing.T, dir string) []string {
 	_ = filepath.WalkDir(dir, func(p string, _ os.DirEntry, _ error) error { out = append(out, p); return nil })
 	return out
 }
+
+// The first match must not hide a second one: this wing's own page of the name
+// exists AND a page of the same name lives elsewhere.
+func TestRun_ASlugUsedInTwoPlacesIsRefusedEvenWhenTheOwnPageIsOneOfThem(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki/web-clip/sources"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "wiki/web-clip/sources/talk.md"), []byte("---\ntitle: Talk\n---\nown"), 0o644))
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki/entities"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "wiki/entities/talk.md"), []byte("---\ntitle: Talk\n---\nan entity"), 0o644))
+
+	_, err := Run(context.Background(), wingCfg("web-clip", sourceEntityConcept()...), dir, "body", "talk", false)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "already used")
+}