Add CustomMessageEntry rendering infrastructure

- Add renderCustomMessage to HookAPI for registering custom renderers
- Add CustomMessageRenderer type and CustomMessageRenderOptions
- Store customMessageRenderers in LoadedHook
- Add getCustomMessageRenderer(customType) to HookRunner
- SessionContext.entries now aligned with messages (same length, corresponding indices)

TUI can now correlate messages with their source entries to identify
custom_message entries and use hook-provided renderers.

packages/coding-agent/src/core/hooks/runner.ts

@@ -5,6 +5,7 @@
 import { spawn } from "node:child_process";
 import type { LoadedHook, SendHandler } from "./loader.js";
 import type {
+	CustomMessageRenderer,
 	ExecOptions,
 	ExecResult,
 	HookError,
@@ -203,6 +204,20 @@ export class HookRunner {
 		return false;
 	}
 
+	/**
+	 * Get a custom message renderer for the given customType.
+	 * Returns the first renderer found across all hooks, or undefined if none.
+	 */
+	getCustomMessageRenderer(customType: string): CustomMessageRenderer | undefined {
+		for (const hook of this.hooks) {
+			const renderer = hook.customMessageRenderers.get(customType);
+			if (renderer) {
+				return renderer;
+			}
+		}
+		return undefined;
+	}
+
 	/**
 	 * Create the event context for handlers.
 	 */