agents: add AGENTS.md + .agents/ scaffolding (wiki/plans/skills stubs)

Adds the root AGENTS.md entry point and minimal .agents/ skeleton
(SOUL, wiki/master-plan+ARCHITECTURE, plans/, skills/) per the
per-repo-agents-convention. Existing handoffs/ untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.agents/wiki/architecture/README.md

@@ -0,0 +1,4 @@
+# Architecture docs
+
+Per-system architecture notes go here as this repo grows beyond a flat artifact store.
+Currently empty — see `../ARCHITECTURE.md` for the current-state summary.