feat(tui,coding-agent): add OSC 9;4 terminal progress indicators

Emit OSC 9;4 progress start/end sequences during agent streaming and
compaction so terminals (iTerm2, WezTerm, Windows Terminal, Kitty, etc.)
show activity in their tab bar.

Made with love for @lucasmeijer

packages/tui/test/virtual-terminal.ts

@@ -100,6 +100,8 @@ export class VirtualTerminal implements Terminal {
 		this.xterm.write(`\x1b]0;${title}\x07`);
 	}
 
+	setProgress(_active: boolean): void {}
+
 	// Test-specific methods not in Terminal interface
 
 	/**