fix(cli): non-zero exit when an agent command emits an error envelope

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

internal/cli/agent.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"errors"
 	"io"
 	"time"
 
@@ -9,6 +10,10 @@ import (
 	"git.dcglab.co.uk/steve/emcli/internal/store"
 )
 
+// errCommandFailed signals that a command emitted an error envelope, so the
+// process should exit non-zero. The JSON envelope is the authoritative detail.
+var errCommandFailed = errors.New("command reported error")
+
 // Mailer is the subset of the IMAP client the agent commands use.
 type Mailer interface {
 	SelectFolder(folder string) (uint32, uint32, error)
@@ -26,7 +31,15 @@ type Deps struct {
 	Out   io.Writer
 }
 
-func (d Deps) emit(e Envelope) error { return e.Write(d.Out) }
+func (d Deps) emit(e Envelope) error {
+	if err := e.Write(d.Out); err != nil {
+		return err // write failure (rare) — propagate as-is
+	}
+	if e.Error {
+		return errCommandFailed
+	}
+	return nil
+}
 
 func (d Deps) audit(account, action, target, result, reason string) {
 	_ = d.Store.Audit(d.Now(), store.AuditEntry{