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>

src/channel.ts

@@ -159,7 +159,10 @@ export const fabricChannelPlugin = createChatChannelPlugin<ResolvedFabricAccount
         cfg?: unknown;
         config?: unknown;
       }) => {
-        // openclaw passes config under cfg or config depending on path
+        // openclaw passes config under cfg or config depending on path.
+        // Note: inbound agent replies go through inbound.ts `deliver`
+        // (where turn coalescing happens). This path is for any direct
+        // outbound sends and posts immediately.
         const cfg = (ctx.cfg ?? ctx.config ?? {}) as AnyCfg;
         try {
           const r = await sendToFabric(cfg, ctx.accountId ?? null, ctx.to, ctx.text);