feat: full-height right agent panel with per-agent last activity

The /agents-panel now fills the whole right column (40% width, 100% height,
top-right anchor). Under each running subagent it shows the agent's most
recent activity read from the tail of its session file: the last tool call
with its query (e.g. web_search: "...") or the last assistant text. Reads are
cached by file size so the 1.5s refresh stays cheap.

src/supervisor.ts

@@ -83,7 +83,7 @@ export class SupervisorIntegration {
       return createAgentPanelComponent(tree, tui, theme, () => { this.agentPanelClose = undefined; done(undefined); });
     }, {
       overlay: true,
-      overlayOptions: { width: "36%", minWidth: 44, maxHeight: "85%", anchor: "top-right", margin: 1, nonCapturing: true, visible: (termWidth) => termWidth >= 100 },
+      overlayOptions: { width: "40%", minWidth: 44, maxHeight: "100%", anchor: "top-right", margin: 1, nonCapturing: true, visible: (termWidth) => termWidth >= 100 },
     });
   }