Refuse config keys that no setting has (closes #419)

A misspelled or unknown config key was accepted silently, so a typo in exclude backed up everything it meant to leave out. Loading now refuses a key that no setting has, at any depth, naming its dotted path and, when it is in the file, its line; keys brought in by ${...} references are checked too. config set refuses an unknown key. A top-level env: section stays accepted, since the config library uses it. Anchors that contain themselves fail with an error.

Model: opus-5-5

internal/cli/config_test.go

@@ -225,7 +225,7 @@ func TestConfigSetIndexUnderMissingKey(t *testing.T) {
 // change after which the config would not load, with an error giving the
 // reason, and leaves the file as it was. On the file `config init` writes,
 // which does not load until storage_url is set, and on one that cannot be
-// read here, only a value of the wrong type is refused.
+// read here, only an unknown key or a value of the wrong type is refused.
 func TestConfigSetKeepsFileThatWouldNotLoad(t *testing.T) {
 	t.Parallel()
 
@@ -237,6 +237,14 @@ func TestConfigSetKeepsFileThatWouldNotLoad(t *testing.T) {
 		value  string
 		reason string
 	}{
+		// Keys that no setting has, on a config that loads and on one
+		// that does not.
+		{configWithoutRecipients, "nosuchkey", "1",
+			`unknown config key "nosuchkey"`},
+		{configWithoutRecipients, "snapshots.home.exlude", "/tmp",
+			`unknown config key "snapshots.home.exlude"`},
+		{defaultConfigText(macOS), "compression_lvl", "9",
+			`unknown config key "compression_lvl"`},
 		// Index 1 of the empty list the missing key gives.
 		{configWithoutRecipients, "age_recipients.1", testRecipient,
 			"index out of range"},
@@ -335,6 +343,39 @@ func TestConfigSetRefusesOnlyItsOwnFailure(t *testing.T) {
 	}
 }
 
+// TestConfigSetWritesEnvSection checks that config set writes a key under
+// the top-level env section, whose keys are not settings, on a config that
+// loads.
+func TestConfigSetWritesEnvSection(t *testing.T) {
+	const name = "VAULTIK_TEST_CONFIG_SET_ENV"
+
+	// Loading the changed config sets the variable; t.Setenv restores it
+	// after the test.
+	t.Setenv(name, "")
+
+	path := filepath.Join(t.TempDir(), "config.yml")
+
+	err := os.WriteFile(path, []byte(configWithoutRecipients), configFileMode)
+	if err != nil {
+		t.Fatalf("write config: %v", err)
+	}
+
+	err = writeConfigSet(ui.NewWithColor(&bytes.Buffer{}, false),
+		path, "env."+name, "value")
+	if err != nil {
+		t.Fatalf("config set env.%s: %v", name, err)
+	}
+
+	data, err := os.ReadFile(path) //nolint:gosec // G304: test-controlled path
+	if err != nil {
+		t.Fatalf("read config: %v", err)
+	}
+
+	if !strings.Contains(string(data), "env:\n  "+name+": value\n") {
+		t.Errorf("config set env.%s wrote:\n%s", name, data)
+	}
+}
+
 // TestConfigSetStringLooksLikeNumber sets string settings to values that
 // YAML reads as numbers or booleans when they are unquoted, and checks that
 // config.Load returns each one unchanged.