fix: RPC mode session management not saving sessions

Since version 0.9.0, RPC mode (--mode rpc) was not saving messages to
session files. The agent.subscribe() call with session management logic
was only present in the TUI renderer after it was refactored.

RPC mode now properly saves sessions just like interactive mode.

Added test for RPC mode session management to prevent regression.

Fixes #83

Thanks @kiliman for reporting this issue!

packages/coding-agent/src/main.ts

@@ -806,10 +806,24 @@ async function runSingleShotMode(
 	}
 }
 
-async function runRpcMode(agent: Agent, _sessionManager: SessionManager): Promise<void> {
-	// Subscribe to all events and output as JSON
-	agent.subscribe((event) => {
+async function runRpcMode(agent: Agent, sessionManager: SessionManager): Promise<void> {
+	// Subscribe to all events and output as JSON (same pattern as tui-renderer)
+	agent.subscribe(async (event) => {
 		console.log(JSON.stringify(event));
+
+		// Save messages to session
+		if (event.type === "message_end") {
+			sessionManager.saveMessage(event.message);
+
+			// Yield to microtask queue to allow agent state to update
+			// (tui-renderer does this implicitly via await handleEvent)
+			await Promise.resolve();
+
+			// Check if we should initialize session now (after first user+assistant exchange)
+			if (sessionManager.shouldInitializeSession(agent.state.messages)) {
+				sessionManager.startSession(agent.state);
+			}
+		}
 	});
 
 	// Listen for JSON input on stdin