feat(chat): consume CR-5 history windowing — server-windowed cold loads + show-earlier backfill

Re-pinned [email protected]>0.10.0 + [email protected]>0.6.1 (reply
frontend-history-windowing-handoff.md); re-mirrored both .dispatch references.

- HistorySync port gains optional { limit?, beforeSeq? } (CR-5 params); the
  app's createHistorySync appends them to GET /conversations/:id.
- COLD-cache fresh load now fetches ?sinceSeq=0&limit=<floor(0.75xL)> — a huge
  conversation no longer ships whole to show 192 chunks. A warm-cache tail sync
  stays unwindowed (windowing a tail that outgrew the limit would leave a
  silent seq gap behind the cache).
- hasEarlier now derives from the [email protected] CONTRACT (1-based gap-free seqs):
  loaded window starting above seq 1 => older history exists — covering both
  locally-trimmed AND server-windowed transcripts (the watermark stays as the
  merge floor only).
- showEarlier(): local cache first; when the cache doesn't reach far enough
  back, backfills the missing older run via ?beforeSeq=<oldestKnown>&limit=
  and persists it (next page-in is local). latestSeq windowed-read caveat is
  satisfied structurally (tail cursor derives from the cache's max seq).
- live-probe: +6 CR-5 checks (seq origin, newest-k ascending, short-chat
  exactness, beforeSeq paging, 400 validation x2). NOT yet run live — backend
  was down at commit time; run pending.
- backend-handoff.md: CR-5 RESOLVED, pins/mirrors current. 602 tests green x2.

1 files changed, 2 insertions, 1 deletions

diff --git a/AGENTS.md b/AGENTS.md
index a4c2f36..bc16ef5 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -139,7 +139,8 @@ streaming; tabs + model selector + DaisyUI/dracula), plus per-conversation cwd +
 context size, cache-warming (+ retention/timer), markdown, smart auto-scroll, multi-client
 live view (subscribe/reconnect + the user prompt on the event stream), and the chat limit
 (bulk quarter-unload past `dispatch.chatLimit`, 75% fresh-load window, show-earlier page-in;
-`core/chunks/trim.ts`; backend CR-5 open for `?limit=`/`?beforeSeq=`). Plan in
+`core/chunks/trim.ts`; CR-5 `?limit=`/`?beforeSeq=` CONSUMED — server-windowed cold loads +
+show-earlier server backfill; `hasOlder` from the 1-based gap-free seq contract). Plan in
 `../arch-rewrite/notes/frontend-design.md` §10.
 
 ## Reports