Load a config that has no age recipient (closes #221)

The README's steps for restoring on another machine failed at the first
command: `config init` wrote a placeholder recipient, and `config.Load`
rejects any recipient that does not parse. `config init` now writes an
empty `age_recipients` list, `config.Load` accepts an empty list, and
`snapshot create` refuses to start without a recipient. A malformed
recipient is still rejected at load.

The recovery-host test now builds its config with `config init` and
`config set` and reads it through `config.Load`, so it imports
`internal/cli`. On a fresh file, `config set age_recipients.0` writes the
list in flow style (`[age1...]`).

Model: opus-5-5

internal/cli/config_test.go

@@ -24,8 +24,10 @@ func TestDefaultConfigTemplateParses(t *testing.T) {
 		t.Fatalf("default config template is not valid YAML: %v", err)
 	}
 
-	if len(cfg.AgeRecipients) != 1 {
-		t.Errorf("expected 1 placeholder age recipient, got %d", len(cfg.AgeRecipients))
+	// A placeholder recipient would fail config.Load, so the template
+	// leaves the list empty.
+	if len(cfg.AgeRecipients) != 0 {
+		t.Errorf("expected no age recipients, got %d", len(cfg.AgeRecipients))
 	}
 
 	home, ok := cfg.Snapshots["home"]
@@ -55,6 +57,43 @@ func TestDefaultConfigTemplateParses(t *testing.T) {
 	}
 }
 
+// TestConfigSetRecipientOnFreshConfig follows the README quickstart: on the
+// file `config init` writes, `config set age_recipients.0` and
+// `config set storage_url` give a config that loads with that recipient.
+func TestConfigSetRecipientOnFreshConfig(t *testing.T) {
+	t.Parallel()
+
+	const recipient = "age1278m9q7dp3chsh2dcy82qk27v047zywyvtxwnj4cvt0z65jw6a7q5dqhfj"
+
+	path := filepath.Join(t.TempDir(), "config.yml")
+
+	err := os.WriteFile(path, []byte(defaultConfigTemplate), configFileMode)
+	if err != nil {
+		t.Fatalf("write config: %v", err)
+	}
+
+	out := ui.NewWithColor(&bytes.Buffer{}, false)
+
+	err = writeConfigSet(out, path, "age_recipients.0", recipient)
+	if err != nil {
+		t.Fatalf("config set age_recipients.0: %v", err)
+	}
+
+	err = writeConfigSet(out, path, "storage_url", "file:///mnt/backups")
+	if err != nil {
+		t.Fatalf("config set storage_url: %v", err)
+	}
+
+	cfg, err := config.Load(path)
+	if err != nil {
+		t.Fatalf("config.Load: %v", err)
+	}
+
+	if len(cfg.AgeRecipients) != 1 || cfg.AgeRecipients[0] != recipient {
+		t.Errorf("age_recipients = %v, want [%s]", cfg.AgeRecipients, recipient)
+	}
+}
+
 const testYAML = `# top comment
 compression_level: 3
 age_recipients: