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/index.js

@@ -5,6 +5,7 @@
 // the OpenClawPluginApi for runtime startup (transport + tools).
 import { defineChannelPluginEntry } from 'openclaw/plugin-sdk/core';
 import { fabricChannelPlugin } from './src/channel.js';
+import { flushAllFabric } from './src/coalesce.js';
 import { FabricInbound } from './src/inbound.js';
 import { listEnabledFabricAccounts } from './src/accounts.js';
 import { registerFabricTools } from './src/tools.js';
@@ -60,7 +61,12 @@ export default defineChannelPluginEntry({
             api.logger.info(`fabric: inbound started for ${accounts.length} account(s)`);
             void syncFabricCommands(client, cfg, accounts, api.logger);
         });
+        // Note: the per-turn coalesce flush happens deterministically in
+        // inbound.ts right after dispatchInboundReplyWithBase resolves (that
+        // is the real "all deliveries done" boundary; the agent_end hook fires
+        // BEFORE deliver()). gateway_stop only flushes any leftover buffer.
         api.on('gateway_stop', () => {
+            void flushAllFabric();
             inbound?.stop();
             inbound = null;
         });