Rename payloads/ to messages/ (per inline note)

The folder note said "rename this folder — its a bad name." Renamed
payloads → messages to better describe what crosses the Ada border.

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

core/AGENTS.md

@@ -8,7 +8,7 @@ working agreements: [`../CLAUDE.md`](../CLAUDE.md).
 The **Ada/SPARK border — D1**. All traffic to the inner brain crosses here
 first. Built with **Alire**. Internal modules under `src/`: `trust` (incl. the
 COBOL invariant-law vault), `organs`, `network`, `protocol`, `daemons`, `core`,
-`types`. These are modules, not separate organs — they share this
+`types`, `messages`. These are modules, not separate organs — they share this
 file.
 
 ## Build & test