fix(ingestion): wrap naked error returns and harden mustJSON helper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ingestion/internal/wiki/inventory.go

@@ -3,6 +3,7 @@ package wiki
 
 import (
 	"bufio"
+	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
@@ -23,7 +24,7 @@ func LoadInventory(brainDir string) (map[PageType][]Entry, error) {
 			continue
 		}
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf("read dir %s: %w", dir, err)
 		}
 		for _, e := range entries {
 			if e.IsDir() || !strings.HasSuffix(e.Name(), ".md") {