fix(tmux): stabilize flaky tests with WaitForShellReady

TestIsAgentRunning and TestEnsureSessionFresh_ZombieSession were flaky
because they checked the pane command immediately after NewSession,
before the shell had fully initialized. Added WaitForShellReady calls
to wait for shell readiness before assertions.

Closes: gt-jzwx

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/tmux/tmux_test.go

@@ -260,6 +260,12 @@ func TestEnsureSessionFresh_ZombieSession(t *testing.T) {
 	}
 	defer func() { _ = tm.KillSession(sessionName) }()
 
+	// Wait for shell to be ready - avoids flaky tests where the pane command
+	// is briefly something other than the shell during initialization
+	if err := tm.WaitForShellReady(sessionName, 2*time.Second); err != nil {
+		t.Fatalf("WaitForShellReady: %v", err)
+	}
+
 	// Verify it's a zombie (not running Claude/node)
 	if tm.IsClaudeRunning(sessionName) {
 		t.Skip("session unexpectedly has Claude running - can't test zombie case")
@@ -332,6 +338,12 @@ func TestIsAgentRunning(t *testing.T) {
 	}
 	defer func() { _ = tm.KillSession(sessionName) }()
 
+	// Wait for shell to be ready - avoids flaky tests where the pane command
+	// is briefly something other than the shell during initialization
+	if err := tm.WaitForShellReady(sessionName, 2*time.Second); err != nil {
+		t.Fatalf("WaitForShellReady: %v", err)
+	}
+
 	// Get the current pane command (should be bash/zsh/etc)
 	cmd, err := tm.GetPaneCommand(sessionName)
 	if err != nil {