style(agent): redesign AgentPane and MarkdownPane for polished readable UI

Tribunal-elected design (S-3-R4, 88% confidence). AgentPane: sans-serif root
font, tool call/result pairing via $derived.by, hook message collapsing,
context window meter, minimal cost bar, session summary styling, two-phase
scroll anchoring, tool-aware truncation, color-mix() softening, container
query responsive margins. MarkdownPane: container query wrapper. Shared
--bterminal-pane-padding-inline CSS variable in catppuccin.css.

v2/src/lib/styles/catppuccin.css

@@ -55,4 +55,7 @@
   --pane-header-height: 32px;
   --pane-gap: 2px;
   --border-radius: 4px;
+
+  /* Pane content padding — shared between AgentPane and MarkdownPane */
+  --bterminal-pane-padding-inline: clamp(0.75rem, 3.5cqi, 2rem);
 }