feat(amlich): add Vietnamese lunar calendar conversion module

/amlich converts duong lich to am lich (defaults to today, Asia/Saigon);
/duonglich converts am lich to duong lich with a nhuan flag for leap
months. Dates accept d, d/m, or d/m/yyyy - missing parts fill from today
in the input's calendar. Conversion is a dependency-free port of Ho Ngoc
Duc's algorithm at UTC+7, with rejection of impossible lunar inputs,
anchored by known Tet/leap-month dates and a 1950-2050 round-trip test.

cmd/server/command_menu_test.go

@@ -58,6 +58,8 @@ func TestCommandDiscovery_AllPublicCommandsHaveSafeMetadata(t *testing.T) {
 	}
 
 	expectedParameters := map[string]string{
+		"amlich":               "[date]",
+		"duonglich":            "<date> [nhuan]",
 		"coin_price":           "<coin>",
 		"coin_topup":           "<usd_amount>",
 		"coin_buy":             "<coin> <usd_to_spend>",