docs: say that the board has two shapes now

F1 described "one lane per game" as a fact about the timeline, which is
now false half the time, and the prefs key space did not list the pref
that decides it. Both say why the merged board sorts with
endingSoonestFirst rather than a bare end date, since that is the part a
later change is most likely to "simplify".

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

AGENTS.md

@@ -113,9 +113,10 @@ src/client/       React app, service worker, manifest
                   useCustom.ts — the reader's own games and events (PRD F13)
                   lens.ts — who sees which rows (focus, outstanding, next-to-expire); pure
                   zoom.ts — the timeline's scale ladder; pure
+                  lanes.ts — how the timeline stacks: a lane per game, or one deadline queue; pure
 scripts/          build-feed.ts, build-static.ts, parse-fixture.ts (offline), refresh-sources.ts (fetches)
 serve.ts          static server + /api/health
-test/             599 tests
+test/             606 tests
 fixtures/<game>/  raw HTML + .expected.json per source — pinned, kept forever
 snapshots/        current page per source, rewritten by refresh — see its README
 ```