fix: resolve ${PWD} in system prompt from session cwd, not process env

expandSystemPrompt fell through to os.Getenv("PWD") which always
returned the process startup directory. Map "PWD" explicitly to the
cwd parameter so the system prompt reflects the session's working
directory.

pkg/agent/core.go

@@ -137,6 +137,8 @@ func expandSystemPrompt(content, cwd string) string {
 	}
 	mapper := func(key string) string {
 		switch key {
+		case "PWD":
+			return cwd
 		case "PRIME_DATE":
 			return time.Now().Format("2006-01-02")
 		case "PRIME_PLATFORM":

pkg/agent/core_test.go

@@ -544,6 +544,14 @@ func TestExpandSystemPrompt_IsGitFalse(t *testing.T) {
 	}
 }
 
+func TestExpandSystemPrompt_PWDUsesCWD(t *testing.T) {
+	dir := t.TempDir()
+	got := expandSystemPrompt("${PWD}", dir)
+	if got != dir {
+		t.Errorf("got %q; want %q", got, dir)
+	}
+}
+
 // --- BuildAgentEnv: missing system prompt ---
 
 func TestBuildAgentEnv_MissingSystemPrompt(t *testing.T) {