fix(depot): exit 64 on unset DEPOT_DIR, count push transport failures

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

internal/depot/run.go

@@ -3,6 +3,7 @@ package depot
 import (
 	"context"
 	"crypto/sha256"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -60,18 +61,30 @@ func printRunUsage(w io.Writer) {
 `)
 }
 
+// errUsage marks errors that are the caller's fault (usage, exit 64) rather
+// than internal/backend failures (exit 70).
+var errUsage = errors.New("usage error")
+
 // resolveBackend builds the named backend, resolving "local" against DEPOT_DIR.
 func resolveBackend(target string, getenv func(string) string, cfg Config) (Backend, error) {
 	root := ""
 	if target == "local" {
 		root = getenv("DEPOT_DIR")
 		if root == "" {
-			return nil, fmt.Errorf("--target local requires DEPOT_DIR to be set")
+			return nil, fmt.Errorf("--target local requires DEPOT_DIR to be set: %w", errUsage)
 		}
 	}
 	return NewBackend(target, root, cfg)
 }
 
+// backendErrExit maps a resolveBackend error onto the exit contract.
+func backendErrExit(err error) int {
+	if errors.Is(err, errUsage) {
+		return exitUsage
+	}
+	return exitInternal
+}
+
 func printSweepStatus(stdout io.Writer, referenced int, res SweepResult) {
 	fmt.Fprintf(stdout, "referenced=%d present=%d missing=%d unconfirmed=%d\n",
 		referenced, len(res.Present), len(res.Missing), len(res.Unconfirmed))
@@ -120,7 +133,7 @@ func runStatus(args []string, stdout, stderr io.Writer, getenv func(string) stri
 	backend, err := resolveBackend(*target, getenv, cfg)
 	if err != nil {
 		fmt.Fprintln(stderr, "depot status:", err)
-		return exitInternal
+		return backendErrExit(err)
 	}
 
 	shaSizes, err := ReferencedSHAs(*manifests)
@@ -199,6 +212,7 @@ func runPush(args []string, stdout, stderr io.Writer, getenv func(string) string
 		}
 		if err := backend.Put(context.Background(), sha, srcPath); err != nil {
 			mu.Lock()
+			failed++
 			if transportErr == nil {
 				transportErr = err
 			}
@@ -318,7 +332,7 @@ func runGet(args []string, stdout, stderr io.Writer, getenv func(string) string)
 	backend, err := resolveBackend(*target, getenv, cfg)
 	if err != nil {
 		fmt.Fprintln(stderr, "depot get:", err)
-		return exitInternal
+		return backendErrExit(err)
 	}
 
 	if err := backend.Get(context.Background(), sha, dest); err != nil {