feat(coding-agent): support session dir env

closes #4027

packages/coding-agent/src/config.ts

@@ -327,6 +327,13 @@ export const VERSION: string = pkg.version || "0.0.0";
 
 // e.g., PI_CODING_AGENT_DIR or TAU_CODING_AGENT_DIR
 export const ENV_AGENT_DIR = `${APP_NAME.toUpperCase()}_CODING_AGENT_DIR`;
+export const ENV_SESSION_DIR = `${APP_NAME.toUpperCase()}_CODING_AGENT_SESSION_DIR`;
+
+export function expandTildePath(path: string): string {
+	if (path === "~") return homedir();
+	if (path.startsWith("~/")) return homedir() + path.slice(1);
+	return path;
+}
 
 const DEFAULT_SHARE_VIEWER_URL = "https://pi.dev/session/";
 
@@ -344,10 +351,7 @@ export function getShareViewerUrl(gistId: string): string {
 export function getAgentDir(): string {
 	const envDir = process.env[ENV_AGENT_DIR];
 	if (envDir) {
-		// Expand tilde to home directory
-		if (envDir === "~") return homedir();
-		if (envDir.startsWith("~/")) return homedir() + envDir.slice(1);
-		return envDir;
+		return expandTildePath(envDir);
 	}
 	return join(homedir(), CONFIG_DIR_NAME, "agent");
 }