feat(pipeline): per-wing sources, shared concepts/entities, one slug one page (#208)

With Config.Wing a source goes to wiki/<wing>/sources/ (wing/hall frontmatter);
concepts and entities stay in the shared wiki/concepts and wiki/entities, so an
entity cited from two wings is ONE page with both sources (the per-wing facts/
layout duplicated it). A source slug already used anywhere but its own wing's
sources/ is refused (slug is the graph identity). wiki.FindSlug added.
Mutation-checked: collision guard, shared inventory, per-wing source path.

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

ingestion/internal/wiki/findslug_test.go

@@ -0,0 +1,52 @@
+package wiki
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func put(t *testing.T, dir, rel string) {
+	t.Helper()
+	full := filepath.Join(dir, rel)
+	require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+	require.NoError(t, os.WriteFile(full, []byte("---\ntitle: x\n---\n"), 0o644))
+}
+
+func TestFindSlug(t *testing.T) {
+	dir := t.TempDir()
+	put(t, dir, "wiki/sources/legacy-src.md")
+	put(t, dir, "wiki/concepts/a-concept.md")
+	put(t, dir, "wiki/entities/an-entity.md")
+	put(t, dir, "wiki/homelab/failures/in-a-hall.md")
+	put(t, dir, "wiki/web-clip/sources/a-clip.md")
+
+	tests := []struct {
+		slug string
+		want string
+	}{
+		{"legacy-src", "wiki/sources/legacy-src.md"},
+		{"a-concept", "wiki/concepts/a-concept.md"},
+		{"an-entity", "wiki/entities/an-entity.md"},
+		{"in-a-hall", "wiki/homelab/failures/in-a-hall.md"},
+		{"a-clip", "wiki/web-clip/sources/a-clip.md"},
+		{"nothing-here", ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.slug, func(t *testing.T) {
+			got, found, err := FindSlug(dir, tt.slug)
+			require.NoError(t, err)
+			assert.Equal(t, tt.want != "", found)
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}
+
+func TestFindSlug_NoWikiDirIsNotAFinding(t *testing.T) {
+	_, found, err := FindSlug(t.TempDir(), "x")
+	require.NoError(t, err)
+	assert.False(t, found)
+}