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>

AGENTS.md

@@ -6,9 +6,11 @@ submodule. The
 Android app uses a thin Java activity and `android-view`; desktop uses winit.
 
 `docs/PLAN.md` is the design source of truth. Read it before structural work
-and update it when a decision changes. Working documents are pruned as work
-lands: preserve current invariants, measurements, and failed hypotheses, not a
-chronicle of completed tasks. Do not create a decisions log.
+and update it when a decision changes. `docs/HANDOFF.md` is where the work in
+flight stands; read it first in a fresh session and keep it current. Working
+documents are pruned as work lands: preserve current invariants, measurements,
+and failed hypotheses, not a chronicle of completed tasks. Do not create a
+decisions log.
 
 ## Architecture