fix: flush block reply coalescer on tool boundaries

When block streaming is enabled with verbose=off, tool blocks are hidden
but their boundary information was lost. Text segments before and after
tool execution would get coalesced into a single message because the
coalescer had no signal that a tool had executed between them.

This adds an onBlockReplyFlush callback that fires on tool_execution_start,
allowing the block reply pipeline to flush pending text before the tool
runs. This preserves natural message boundaries even when tools are hidden.

Fixes the issue where:
  text → [hidden tool] → text → rendered as one merged message

Now correctly renders as:
  text → [hidden tool] → text → two separate messages

Co-diagnosed-by: Krill (Discord assistant)

src/auto-reply/reply/agent-runner.ts

@@ -643,6 +643,12 @@ export async function runReplyAgent(params: {
                       blockReplyPipeline?.enqueue(blockPayload);
                     }
                   : undefined,
+              onBlockReplyFlush:
+                blockStreamingEnabled && blockReplyPipeline
+                  ? async () => {
+                      await blockReplyPipeline.flush({ force: true });
+                    }
+                  : undefined,
               shouldEmitToolResult,
               onToolResult: opts?.onToolResult
                 ? (payload) => {