fix(ui): friendly name and icon for the get_ast_outline tool card

get_ast_outline was the one registry tool with no Tool::display_name and no
Tool::icon, so its chat card fell back to the raw function id and the generic
Filesystem glyph. It now declares "Code Outline" plus a new semantic icon key
'outline', mapped frontend-side to bi-list-nested with its own accent var in
both themes.

web/css/variables.css

@@ -18,6 +18,7 @@
   --tool-read:           #0891b2;
   --tool-list:           #7c3aed;
   --tool-search:         #9333ea;
+  --tool-outline:        #0d9488;
   --tool-shell:          #475569;
   --tool-subagent:       #0d9488;
   --tool-image:          #db2777;
@@ -85,6 +86,7 @@
   --tool-read:           #22d3ee;
   --tool-list:           #a78bfa;
   --tool-search:         #c084fc;
+  --tool-outline:        #2dd4bf;
   --tool-shell:          #94a3b8;
   --tool-subagent:       #2dd4bf;
   --tool-image:          #f472b6;