Replace custom tool dispose() with shutdown session event

Breaking change: CustomAgentTool.dispose() removed. Use onSession with
reason 'shutdown' instead for cleanup.

- Add 'shutdown' to SessionEvent.reason for custom tools
- Remove dispose() method from CustomAgentTool interface
- Make emitToolSessionEvent() public on AgentSession
- Emit shutdown event to tools in InteractiveMode.shutdown()
- Update custom-tools.md with new API and examples

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -1239,7 +1239,7 @@ export class InteractiveMode {
 
 	/**
 	 * Gracefully shutdown the agent.
-	 * Emits shutdown event to hooks, then exits.
+	 * Emits shutdown event to hooks and tools, then exits.
 	 */
 	private async shutdown(): Promise<void> {
 		// Emit shutdown event to hooks
@@ -1250,6 +1250,9 @@ export class InteractiveMode {
 			});
 		}
 
+		// Emit shutdown event to custom tools
+		await this.session.emitToolSessionEvent("shutdown");
+
 		this.stop();
 		process.exit(0);
 	}