fix: TUI overflow, scrollable header, tool output, git branch

- Fixed: chat content no longer overflows past allocated height.
  Lines are measured for physical width and hard-truncated to
  exactly the chat area height. Input + status bar always visible.
- Header scrolls with chat (not pinned), only input/status fixed
- Git branch in status bar (green, via git rev-parse)
- Alt screen mode — terminal scrollback disabled
- Mouse wheel + PgUp/PgDown scroll within TUI
- New EventToolResult: tool output as dimmed indented block
- Separator lines above/below input, no status bar backgrounds

internal/stream/event.go

@@ -16,6 +16,7 @@ const (
 	EventToolCallStart
 	EventToolCallDelta
 	EventToolCallDone
+	EventToolResult // tool execution output
 	EventUsage
 	EventError
 )
@@ -32,6 +33,8 @@ func (et EventType) String() string {
 		return "tool_call_delta"
 	case EventToolCallDone:
 		return "tool_call_done"
+	case EventToolResult:
+		return "tool_result"
 	case EventUsage:
 		return "usage"
 	case EventError:
@@ -56,6 +59,10 @@ type Event struct {
 	ArgDelta     string          // partial JSON fragment
 	Args         json.RawMessage // complete arguments (on Done)
 
+	// ToolResult: tool name + output
+	ToolName   string
+	ToolOutput string
+
 	// Usage
 	Usage *message.Usage