fix: improve error messages for flake metadata command

Capture stderr from nix flake metadata to show the actual error
message instead of just "exit status 1".

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

collector/flake.go

@@ -3,6 +3,7 @@ package collector
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"log/slog"
 	"os"
 	"os/exec"
@@ -189,12 +190,15 @@ func fetchFlakeMetadata(flakeURL string) (*flakeMetadata, error) {
 	cmd := exec.CommandContext(ctx, "nix", "flake", "metadata", "--json", flakeURL)
 	output, err := cmd.Output()
 	if err != nil {
-		return nil, err
+		if exitErr, ok := err.(*exec.ExitError); ok {
+			return nil, fmt.Errorf("nix flake metadata failed: %s", string(exitErr.Stderr))
+		}
+		return nil, fmt.Errorf("nix flake metadata failed: %w", err)
 	}
 
 	var data flakeMetadata
 	if err := json.Unmarshal(output, &data); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("failed to parse flake metadata: %w", err)
 	}
 
 	return &data, nil