wip: add long message helpers and payload type controls

- add centralized message validation errors
- switch command handlers to return error
- add explicit long plain-text reply helpers
- support strict payload type policy with debug logging
- document versioning and changelog workflow

bot_opts_test.go

@@ -45,3 +45,12 @@ func TestLoadPrefixesFromEnvDropsEmptyValues(t *testing.T) {
 		t.Fatalf("unexpected prefixes: got %v want %v", got, want)
 	}
 }
+
+func TestLoadOptsFromEnvReadsStrictPayloadType(t *testing.T) {
+	t.Setenv("STRICT_PAYLOAD_TYPE", "true")
+
+	opts := LoadOptsFromEnv()
+	if !opts.StrictPayloadType {
+		t.Fatal("expected StrictPayloadType to be enabled")
+	}
+}