feat(fabric): coalesce a split agent turn into ONE message (deterministic)

OpenClaw delivers an agent turn whose blocks are text -> thinking/tool
-> text via multiple inbound deliver() calls (a non-text block is a
delivery boundary), so one turn became N Fabric messages.

Fix: buffer deliver() segments per channel (src/coalesce.ts) and flush
them as ONE postMessage at a deterministic boundary — the finally after
dispatchInboundReplyWithBase() resolves, which provably runs only after
every deliver() of the turn (verified: deliver,deliver -> dispatch
returned -> flush). No hooks, no timers, no idle guessing. The
agent_end hook was rejected: it fires BEFORE deliver(). gateway_stop
flushes any leftover; a long safety timeout is a leak-guard only.
channels.fabric.coalesce=false restores raw per-segment posting.

Verified on local openclaw + Fabric with a fake text/thinking/text
model: single trigger -> exactly one merged message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dist/fabric/src/accounts.js

@@ -13,6 +13,11 @@ function section(cfg) {
 export function resolveCommandsSyncKey(cfg) {
     return (section(cfg).commandsSyncKey ?? '').trim();
 }
+// Whether to coalesce a split agent turn into one Fabric message
+// (channel-level). Default true.
+export function resolveCoalesce(cfg) {
+    return (cfg.channels?.fabric ?? {}).coalesce !== false;
+}
 export function listFabricAccountIds(cfg) {
     const accts = section(cfg).accounts ?? {};
     const ids = Object.keys(accts);