feat: warn/critical context bar at 20/40 percent

The context bar colored at 40/70 percent, which on a 1M-token window only
turned yellow at 400k and red at 700k - far too late to warn before the
window fills. Percentage thresholds stay model-independent while mapping to
the intended 200k/400k marks on the current 1M window.

assets/cship.toml

@@ -24,9 +24,9 @@ empty_char         = "░"
 format             = "[$symbol$value]($style)"
 width              = 10
 style              = "fg:#7dcfff"
-warn_threshold     = 40.0
+warn_threshold     = 20.0
 warn_style         = "yellow"
-critical_threshold = 70.0
+critical_threshold = 40.0
 critical_style     = "bold red"
 
 # $value is emitted inside a §T§…§ marker so the statusline wrapper can