feat(coding-agent): implement hooks system

- Add hooks infrastructure in core/hooks/ (loader, runner, types)
- HookUIContext interface with mode-specific implementations
- Interactive mode: TUI-based selector/input/confirm dialogs
- RPC mode: JSON protocol for hook UI requests/responses
- Print mode: no-op UI context (hooks run but can't prompt)
- AgentSession.branch() now async, returns { selectedText, skipped }
- Settings: hooks[] and hookTimeout configuration
- Export hook types from package for hook authors

Based on PR #147 proposal, adapted for new architecture.

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

@@ -0,0 +1,138 @@
+/**
+ * Hook loader - loads TypeScript hook modules using jiti.
+ */
+
+import * as os from "node:os";
+import * as path from "node:path";
+import { createJiti } from "jiti";
+import type { HookAPI, HookFactory } from "./types.js";
+
+/**
+ * Generic handler function type.
+ */
+type HandlerFn = (...args: unknown[]) => Promise<unknown>;
+
+/**
+ * Registered handlers for a loaded hook.
+ */
+export interface LoadedHook {
+	/** Original path from config */
+	path: string;
+	/** Resolved absolute path */
+	resolvedPath: string;
+	/** Map of event type to handler functions */
+	handlers: Map<string, HandlerFn[]>;
+}
+
+/**
+ * Result of loading hooks.
+ */
+export interface LoadHooksResult {
+	/** Successfully loaded hooks */
+	hooks: LoadedHook[];
+	/** Errors encountered during loading */
+	errors: Array<{ path: string; error: string }>;
+}
+
+/**
+ * Expand path with ~ support.
+ */
+function expandPath(p: string): string {
+	if (p.startsWith("~/")) {
+		return path.join(os.homedir(), p.slice(2));
+	}
+	if (p.startsWith("~")) {
+		return path.join(os.homedir(), p.slice(1));
+	}
+	return p;
+}
+
+/**
+ * Resolve hook path.
+ * - Absolute paths used as-is
+ * - Paths starting with ~ expanded to home directory
+ * - Relative paths resolved from cwd
+ */
+function resolveHookPath(hookPath: string, cwd: string): string {
+	const expanded = expandPath(hookPath);
+
+	if (path.isAbsolute(expanded)) {
+		return expanded;
+	}
+
+	// Relative paths resolved from cwd
+	return path.resolve(cwd, expanded);
+}
+
+/**
+ * Create a HookAPI instance that collects handlers.
+ */
+function createHookAPI(handlers: Map<string, HandlerFn[]>): HookAPI {
+	return {
+		on(event: string, handler: HandlerFn): void {
+			const list = handlers.get(event) ?? [];
+			list.push(handler);
+			handlers.set(event, list);
+		},
+	} as HookAPI;
+}
+
+/**
+ * Load a single hook module using jiti.
+ */
+async function loadHook(hookPath: string, cwd: string): Promise<{ hook: LoadedHook | null; error: string | null }> {
+	const resolvedPath = resolveHookPath(hookPath, cwd);
+
+	try {
+		// Create jiti instance for TypeScript/ESM loading
+		const jiti = createJiti(import.meta.url);
+
+		// Import the module
+		const module = await jiti.import(resolvedPath, { default: true });
+		const factory = module as HookFactory;
+
+		if (typeof factory !== "function") {
+			return { hook: null, error: "Hook must export a default function" };
+		}
+
+		// Create handlers map and API
+		const handlers = new Map<string, HandlerFn[]>();
+		const api = createHookAPI(handlers);
+
+		// Call factory to register handlers
+		factory(api);
+
+		return {
+			hook: { path: hookPath, resolvedPath, handlers },
+			error: null,
+		};
+	} catch (err) {
+		const message = err instanceof Error ? err.message : String(err);
+		return { hook: null, error: `Failed to load hook: ${message}` };
+	}
+}
+
+/**
+ * Load all hooks from configuration.
+ * @param paths - Array of hook file paths
+ * @param cwd - Current working directory for resolving relative paths
+ */
+export async function loadHooks(paths: string[], cwd: string): Promise<LoadHooksResult> {
+	const hooks: LoadedHook[] = [];
+	const errors: Array<{ path: string; error: string }> = [];
+
+	for (const hookPath of paths) {
+		const { hook, error } = await loadHook(hookPath, cwd);
+
+		if (error) {
+			errors.push({ path: hookPath, error });
+			continue;
+		}
+
+		if (hook) {
+			hooks.push(hook);
+		}
+	}
+
+	return { hooks, errors };
+}