Merge hooks and custom-tools into unified extensions system (#454)

Breaking changes:
- Settings: 'hooks' and 'customTools' arrays replaced with 'extensions'
- CLI: '--hook' and '--tool' flags replaced with '--extension' / '-e'
- API: HookMessage renamed to CustomMessage, role 'hookMessage' to 'custom'
- API: FileSlashCommand renamed to PromptTemplate
- API: discoverSlashCommands() renamed to discoverPromptTemplates()
- Directories: commands/ renamed to prompts/ for prompt templates

Migration:
- Session version bumped to 3 (auto-migrates v2 sessions)
- Old 'hookMessage' role entries converted to 'custom'

Structural changes:
- src/core/hooks/ and src/core/custom-tools/ merged into src/core/extensions/
- src/core/slash-commands.ts renamed to src/core/prompt-templates.ts
- examples/hooks/ and examples/custom-tools/ merged into examples/extensions/
- docs/hooks.md and docs/custom-tools.md merged into docs/extensions.md

New test coverage:
- test/extensions-runner.test.ts (10 tests)
- test/extensions-discovery.test.ts (26 tests)
- test/prompt-templates.test.ts

packages/coding-agent/src/main.ts

@@ -16,11 +16,9 @@ import { selectSession } from "./cli/session-picker.js";
 import { CONFIG_DIR_NAME, getAgentDir, getModelsPath, VERSION } from "./config.js";
 import type { AgentSession } from "./core/agent-session.js";
 
-import type { LoadedCustomTool } from "./core/custom-tools/index.js";
 import { createEventBus } from "./core/event-bus.js";
 import { exportFromFile } from "./core/export-html/index.js";
-import { discoverAndLoadHooks } from "./core/hooks/index.js";
-import type { HookUIContext } from "./core/index.js";
+import { discoverAndLoadExtensions, type ExtensionUIContext, type LoadedExtension } from "./core/extensions/index.js";
 import type { ModelRegistry } from "./core/model-registry.js";
 import { resolveModelScope, type ScopedModel } from "./core/model-resolver.js";
 import { type CreateAgentSessionOptions, createAgentSession, discoverAuthStorage, discoverModels } from "./core/sdk.js";
@@ -62,13 +60,13 @@ async function runInteractiveMode(
 	migratedProviders: string[],
 	versionCheckPromise: Promise<string | undefined>,
 	initialMessages: string[],
-	customTools: LoadedCustomTool[],
-	setToolUIContext: (uiContext: HookUIContext, hasUI: boolean) => void,
+	extensions: LoadedExtension[],
+	setExtensionUIContext: (uiContext: ExtensionUIContext, hasUI: boolean) => void,
 	initialMessage?: string,
 	initialImages?: ImageContent[],
 	fdPath: string | undefined = undefined,
 ): Promise<void> {
-	const mode = new InteractiveMode(session, version, changelogMarkdown, customTools, setToolUIContext, fdPath);
+	const mode = new InteractiveMode(session, version, changelogMarkdown, extensions, setExtensionUIContext, fdPath);
 
 	await mode.init();
 
@@ -214,7 +212,7 @@ function buildSessionOptions(
 	scopedModels: ScopedModel[],
 	sessionManager: SessionManager | undefined,
 	modelRegistry: ModelRegistry,
-	preloadedHooks?: import("./core/hooks/index.js").LoadedHook[],
+	preloadedExtensions?: LoadedExtension[],
 ): CreateAgentSessionOptions {
 	const options: CreateAgentSessionOptions = {};
 
@@ -273,14 +271,9 @@ function buildSessionOptions(
 		options.skills = [];
 	}
 
-	// Pre-loaded hooks (from early CLI flag discovery)
-	if (preloadedHooks && preloadedHooks.length > 0) {
-		options.preloadedHooks = preloadedHooks;
-	}
-
-	// Additional custom tool paths from CLI
-	if (parsed.customTools && parsed.customTools.length > 0) {
-		options.additionalCustomToolPaths = parsed.customTools;
+	// Pre-loaded extensions (from early CLI flag discovery)
+	if (preloadedExtensions && preloadedExtensions.length > 0) {
+		options.preloadedExtensions = preloadedExtensions;
 	}
 
 	return options;
@@ -297,35 +290,35 @@ export async function main(args: string[]) {
 	const modelRegistry = discoverModels(authStorage);
 	time("discoverModels");
 
-	// First pass: parse args to get --hook paths
+	// First pass: parse args to get --extension paths
 	const firstPass = parseArgs(args);
 	time("parseArgs-firstPass");
 
-	// Early load hooks to discover their CLI flags
+	// Early load extensions to discover their CLI flags
 	const cwd = process.cwd();
 	const agentDir = getAgentDir();
 	const eventBus = createEventBus();
-	const hookPaths = firstPass.hooks ?? [];
-	const { hooks: loadedHooks } = await discoverAndLoadHooks(hookPaths, cwd, agentDir, eventBus);
-	time("discoverHookFlags");
+	const extensionPaths = firstPass.extensions ?? [];
+	const { extensions: loadedExtensions } = await discoverAndLoadExtensions(extensionPaths, cwd, agentDir, eventBus);
+	time("discoverExtensionFlags");
 
-	// Collect all hook flags
-	const hookFlags = new Map<string, { type: "boolean" | "string" }>();
-	for (const hook of loadedHooks) {
-		for (const [name, flag] of hook.flags) {
-			hookFlags.set(name, { type: flag.type });
+	// Collect all extension flags
+	const extensionFlags = new Map<string, { type: "boolean" | "string" }>();
+	for (const ext of loadedExtensions) {
+		for (const [name, flag] of ext.flags) {
+			extensionFlags.set(name, { type: flag.type });
 		}
 	}
 
-	// Second pass: parse args with hook flags
-	const parsed = parseArgs(args, hookFlags);
+	// Second pass: parse args with extension flags
+	const parsed = parseArgs(args, extensionFlags);
 	time("parseArgs");
 
-	// Pass flag values to hooks
+	// Pass flag values to extensions
 	for (const [name, value] of parsed.unknownFlags) {
-		for (const hook of loadedHooks) {
-			if (hook.flags.has(name)) {
-				hook.setFlagValue(name, value);
+		for (const ext of loadedExtensions) {
+			if (ext.flags.has(name)) {
+				ext.setFlagValue(name, value);
 			}
 		}
 	}
@@ -401,7 +394,7 @@ export async function main(args: string[]) {
 		sessionManager = SessionManager.open(selectedPath);
 	}
 
-	const sessionOptions = buildSessionOptions(parsed, scopedModels, sessionManager, modelRegistry, loadedHooks);
+	const sessionOptions = buildSessionOptions(parsed, scopedModels, sessionManager, modelRegistry, loadedExtensions);
 	sessionOptions.authStorage = authStorage;
 	sessionOptions.modelRegistry = modelRegistry;
 	sessionOptions.eventBus = eventBus;
@@ -416,7 +409,7 @@ export async function main(args: string[]) {
 	}
 
 	time("buildSessionOptions");
-	const { session, customToolsResult, modelFallbackMessage } = await createAgentSession(sessionOptions);
+	const { session, extensionsResult, modelFallbackMessage } = await createAgentSession(sessionOptions);
 	time("createAgentSession");
 
 	if (!isInteractive && !session.model) {
@@ -469,8 +462,8 @@ export async function main(args: string[]) {
 			migratedProviders,
 			versionCheckPromise,
 			parsed.messages,
-			customToolsResult.tools,
-			customToolsResult.setUIContext,
+			extensionsResult.extensions,
+			extensionsResult.setUIContext,
 			initialMessage,
 			initialImages,
 			fdPath,