add custom prompts with PS1 and PS2

PS1 and PS2 support the backslash escapes of bash (\u \h \w \W \$ \t \j
\e \[ \] ...) and expand $VAR, ${...}, $(...) and $((...)) before each
prompt. Values of escapes and expansions are sanitized. Lines before the
last one and non-SGR escape sequences are written before readline's
prompt. Without PS1, the default prompt is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main_test.go

@@ -399,6 +399,7 @@ func TestExecInput_CdDashNoPrevious(t *testing.T) {
 // =============================================================================
 
 func TestGetPrompt_Format(t *testing.T) {
+	unsetPS1(t)
 	prompt := getPrompt()
 
 	// Should contain username@hostname:path $ pattern
@@ -414,6 +415,7 @@ func TestGetPrompt_Format(t *testing.T) {
 }
 
 func TestGetPrompt_IncludesHome(t *testing.T) {
+	unsetPS1(t)
 	// Save and restore current directory
 	origDir, _ := os.Getwd()
 	defer os.Chdir(origDir)
@@ -1037,6 +1039,7 @@ func TestSanitize(t *testing.T) {
 }
 
 func TestGetPrompt_EscapesControlCharacters(t *testing.T) {
+	unsetPS1(t)
 	t.Setenv("NO_COLOR", "1")
 	origDir, _ := os.Getwd()
 	defer os.Chdir(origDir)