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>

README.md

@@ -72,6 +72,12 @@ Two ways, both write the same identity registry the transport reads:
   `FABRIC_BACKEND_GUILD_COMMANDS_SYNC_KEY` (Guild C-2). Read it from the
   guild with `docker exec fabric-backend-guild node dist/cli/print-commands-sync-key.js`.
   Sourced from config only — never from the environment.
+- `channels.fabric.coalesce` — default `true`. OpenClaw splits one agent
+  turn whose blocks are `text → thinking/tool → text` into multiple
+  `deliver()` calls; this buffers them and posts ONE Fabric message at the
+  deterministic turn boundary (right after the inbound reply dispatch
+  resolves — no hooks, no timers, no idle guessing). `false` = raw
+  per-segment posting.
 - `channels.fabric.accounts.<agentId>` = `{ fabricApiKey, enabled }`
   (**agent = account**; the account id is the OpenClaw agentId)
 - plugin `identityFilePath` — default `~/.openclaw/fabric-identity.json`