feat(v2): add agent tree, status bar, notifications, settings dialog (Phase 5)

Agent tree visualization (SVG) with horizontal layout and bezier edges.
Global status bar with pane counts, active agents pulse, token/cost totals.
Toast notification system with auto-dismiss and agent dispatcher integration.
Settings dialog with SQLite persistence for shell, cwd, and max panes.
Keyboard shortcuts: Ctrl+W close pane, Ctrl+, open settings.

v2/src/lib/utils/agent-tree.ts

@@ -0,0 +1,88 @@
+// Agent tree builder — constructs hierarchical tree from agent messages
+// Subagents are identified by parent_tool_use_id on their messages
+
+import type { AgentMessage, ToolCallContent, CostContent } from '../adapters/sdk-messages';
+
+export interface AgentTreeNode {
+  id: string;
+  label: string;
+  toolName?: string;
+  status: 'running' | 'done' | 'error';
+  costUsd: number;
+  tokens: number;
+  children: AgentTreeNode[];
+}
+
+/**
+ * Build a tree from a flat list of agent messages.
+ * Root node represents the main agent session.
+ * Child nodes represent tool_use calls (potential subagents).
+ */
+export function buildAgentTree(
+  sessionId: string,
+  messages: AgentMessage[],
+  sessionStatus: string,
+  sessionCost: number,
+  sessionTokens: number,
+): AgentTreeNode {
+  const root: AgentTreeNode = {
+    id: sessionId,
+    label: sessionId.slice(0, 8),
+    status: sessionStatus === 'running' || sessionStatus === 'starting' ? 'running' :
+            sessionStatus === 'error' ? 'error' : 'done',
+    costUsd: sessionCost,
+    tokens: sessionTokens,
+    children: [],
+  };
+
+  // Map tool_use_id -> node for nesting
+  const toolNodes = new Map<string, AgentTreeNode>();
+
+  for (const msg of messages) {
+    if (msg.type === 'tool_call') {
+      const tc = msg.content as ToolCallContent;
+      const node: AgentTreeNode = {
+        id: tc.toolUseId,
+        label: tc.name,
+        toolName: tc.name,
+        status: 'running', // will be updated by result
+        costUsd: 0,
+        tokens: 0,
+        children: [],
+      };
+      toolNodes.set(tc.toolUseId, node);
+
+      if (msg.parentId) {
+        // This is a subagent tool call — attach to parent tool node
+        const parent = toolNodes.get(msg.parentId);
+        if (parent) {
+          parent.children.push(node);
+        } else {
+          root.children.push(node);
+        }
+      } else {
+        root.children.push(node);
+      }
+    }
+
+    if (msg.type === 'tool_result') {
+      const tr = msg.content as { toolUseId: string };
+      const node = toolNodes.get(tr.toolUseId);
+      if (node) {
+        node.status = 'done';
+      }
+    }
+  }
+
+  return root;
+}
+
+/** Flatten tree to get total count of nodes */
+export function countTreeNodes(node: AgentTreeNode): number {
+  return 1 + node.children.reduce((sum, c) => sum + countTreeNodes(c), 0);
+}
+
+/** Aggregate cost across a subtree */
+export function subtreeCost(node: AgentTreeNode): number {
+  return node.costUsd + node.children.reduce((sum, c) => sum + subtreeCost(c), 0);
+}