feat(chat): chat limit — bulk quarter-unload, 75% fresh-load window, show-earlier page-in

Long transcripts no longer grow unbounded: past the chat limit (default 256
chunks, localStorage dispatch.chatLimit) the oldest ceil(limit/4) committed
chunks are unloaded in ONE bulk pass — never one-per-delta (old Dispatch's
scroll-jump-per-step bug) — and only while the reader is stuck to the bottom
(scrolled-up readers defer the trim; it catches up in whole quarters). A fresh
page load windows to the newest floor(0.75*limit). Unloading is purely local
(IndexedDB cache + server keep everything); a hiddenBeforeSeq watermark keeps
history merges from resurrecting unloaded chunks, and a 'Show earlier messages'
affordance pages a quarter back in from the cache with scroll-anchor
preservation. Thinking-collapse render keys stay stable across trims via a
hiddenThinkingCount ordinal base.

- core/chunks/trim.ts: pure policy (trim/window/restore/normalize) + tests
- chat store: chatLimit + canUnload deps, windowed load, showEarlier()
- composition root: dispatch.chatLimit localStorage knob + unload gate wired
  to smart-scroll isAtBottom()
- backend CR-5 OPENED (not a blocker): ?limit=/?beforeSeq= on
  GET /conversations/:id (courier backend-handoff-chat-limit.md)
- scripts/live-probe.ts: fix pre-existing stale TurnMetricsEntry reads
  (m1.usage -> total.usage) that crashed the probe; 17/17 live checks pass

1 files changed, 4 insertions, 2 deletions

diff --git a/AGENTS.md b/AGENTS.md
index 3f7d428..a4c2f36 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -136,8 +136,10 @@ browser effects, run a LIVE probe:
 ## Status
 Slices 1–3 DONE + committed (surface system + WS; conversation transcript cache + delta
 streaming; tabs + model selector + DaisyUI/dracula), plus per-conversation cwd + LSP view,
-context size, cache-warming (+ retention/timer), markdown, smart auto-scroll, and
-multi-client live view (subscribe/reconnect + the user prompt on the event stream). Plan in
+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
 `../arch-rewrite/notes/frontend-design.md` §10.
 
 ## Reports