feat: US-018 - Add SubagentTool with ToolResult support

Created new SubagentTool for synchronous subagent execution:
- Implements Tool interface with Name(), Description(), Parameters(), SetContext(), Execute()
- Returns ToolResult with ForUser (summary), ForLLM (full details), Silent=false, Async=false
- Registered in AgentLoop with context support
- Comprehensive test file subagent_tool_test.go with 9 passing tests

Acceptance criteria met:
- ForUser contains subagent output summary (truncated to 500 chars)
- ForLLM contains full execution details with label and result
- Typecheck passes (go build ./... succeeds)
- go test ./pkg/tools -run TestSubagentTool passes (all 9 tests pass)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/loop.go

@@ -84,6 +84,10 @@ func NewAgentLoop(cfg *config.Config, msgBus *bus.MessageBus, provider providers
 	spawnTool := tools.NewSpawnTool(subagentManager)
 	toolsRegistry.Register(spawnTool)
 
+	// Register subagent tool (synchronous execution)
+	subagentTool := tools.NewSubagentTool(subagentManager)
+	toolsRegistry.Register(subagentTool)
+
 	// Register edit file tool
 	editFileTool := tools.NewEditFileTool(workspace)
 	toolsRegistry.Register(editFileTool)
@@ -500,6 +504,11 @@ func (al *AgentLoop) updateToolContexts(channel, chatID string) {
 			st.SetContext(channel, chatID)
 		}
 	}
+	if tool, ok := al.tools.Get("subagent"); ok {
+		if st, ok := tool.(tools.ContextualTool); ok {
+			st.SetContext(channel, chatID)
+		}
+	}
 }
 
 // maybeSummarize triggers summarization if the session history exceeds thresholds.