feat(KB-123): add agent role tracking and expanded logging to agent system

- Extend AgentLogEntry with agent field and new event types (thinking, tool_end)
- Expand AgentLogger with thinking, tool_end callbacks and agent role support
- Wire new logging callbacks in createKbAgent and all agent call-sites (executor, merger, reviewer, triage, pi)
- Update AgentLogViewer with agent role badges and rendering for new entry types
- Export AgentRole and AgentLogType from core package and add tests for new functionality

packages/engine/src/merger.ts

@@ -221,6 +221,7 @@ export async function aiMergeTask(
   const agentLogger = new AgentLogger({
     store,
     taskId,
+    agent: "merger",
     // Merger callbacks don't include taskId — wrap to match AgentLogger signature
     onAgentText: options.onAgentText
       ? (_id, delta) => options.onAgentText!(delta)
@@ -236,7 +237,9 @@ export async function aiMergeTask(
     systemPrompt: buildMergeSystemPrompt(includeTaskId),
     tools: "coding",
     onText: agentLogger.onText,
+    onThinking: agentLogger.onThinking,
     onToolStart: agentLogger.onToolStart,
+    onToolEnd: agentLogger.onToolEnd,
     defaultProvider: settings.defaultProvider,
     defaultModelId: settings.defaultModelId,
     defaultThinkingLevel: settings.defaultThinkingLevel,