Name the handoff doc in AGENTS.md and generalise it to HANDOFF.md

A fresh session had no way to learn the handoff existed: AGENTS.md named
only docs/PLAN.md, and the filename tied the document to one piece of work.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/LAYOUT.md

@@ -6,7 +6,7 @@ without reading that result does not make the parent's size depend on it.
 
 §1 and §2 have landed in Iris (#16 and #18) and the notes below have been
 brought to what shipped rather than what was proposed; §3 to §6 describe the
-same design as it stands. `docs/IRIS_EXTRACTION_HANDOFF.md` has the invariants
+same design as it stands. `docs/HANDOFF.md` has the invariants
 the code now rests on and what is still to do.
 
 ## Design