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/index.ts

@@ -33,25 +33,53 @@ export {
 	serializeConversation,
 	shouldCompact,
 } from "./core/compaction/index.js";
-// Custom tools
-export type {
-	AgentToolUpdateCallback,
-	CustomTool,
-	CustomToolAPI,
-	CustomToolContext,
-	CustomToolFactory,
-	CustomToolSessionEvent,
-	CustomToolsLoadResult,
-	CustomToolUIContext,
-	ExecResult,
-	LoadedCustomTool,
-	RenderResultOptions,
-} from "./core/custom-tools/index.js";
-export { discoverAndLoadCustomTools, loadCustomTools } from "./core/custom-tools/index.js";
 export { createEventBus, type EventBus, type EventBusController } from "./core/event-bus.js";
-export type * from "./core/hooks/index.js";
-// Hook system types and type guards
+// Extension system
+export type {
+	AgentEndEvent,
+	AgentStartEvent,
+	AgentToolResult,
+	AgentToolUpdateCallback,
+	BeforeAgentStartEvent,
+	ContextEvent,
+	ExecOptions,
+	ExecResult,
+	ExtensionAPI,
+	ExtensionCommandContext,
+	ExtensionContext,
+	ExtensionError,
+	ExtensionEvent,
+	ExtensionFactory,
+	ExtensionFlag,
+	ExtensionHandler,
+	ExtensionShortcut,
+	ExtensionUIContext,
+	LoadExtensionsResult,
+	LoadedExtension,
+	MessageRenderer,
+	MessageRenderOptions,
+	RegisteredCommand,
+	RegisteredTool,
+	SessionBeforeBranchEvent,
+	SessionBeforeCompactEvent,
+	SessionBeforeSwitchEvent,
+	SessionBeforeTreeEvent,
+	SessionBranchEvent,
+	SessionCompactEvent,
+	SessionShutdownEvent,
+	SessionStartEvent,
+	SessionSwitchEvent,
+	SessionTreeEvent,
+	ToolCallEvent,
+	ToolDefinition,
+	ToolRenderResultOptions,
+	ToolResultEvent,
+	TurnEndEvent,
+	TurnStartEvent,
+} from "./core/extensions/index.js";
 export {
+	discoverAndLoadExtensions,
+	ExtensionRunner,
 	isBashToolResult,
 	isEditToolResult,
 	isFindToolResult,
@@ -59,7 +87,12 @@ export {
 	isLsToolResult,
 	isReadToolResult,
 	isWriteToolResult,
-} from "./core/hooks/index.js";
+	loadExtensions,
+	wrapRegisteredTool,
+	wrapRegisteredTools,
+	wrapToolsWithExtensions,
+	wrapToolWithExtensions,
+} from "./core/extensions/index.js";
 export { convertToLlm } from "./core/messages.js";
 export { ModelRegistry } from "./core/model-registry.js";
 // SDK for programmatic usage
@@ -83,17 +116,12 @@ export {
 	// Discovery
 	discoverAuthStorage,
 	discoverContextFiles,
-	discoverCustomTools,
-	discoverHooks,
+	discoverExtensions,
 	discoverModels,
+	discoverPromptTemplates,
 	discoverSkills,
-	discoverSlashCommands,
-	type FileSlashCommand,
-	// Hook types
-	type HookAPI,
-	type HookContext,
-	type HookFactory,
 	loadSettings,
+	type PromptTemplate,
 	// Pre-built tools (use process.cwd())
 	readOnlyTools,
 } from "./core/sdk.js";
@@ -159,9 +187,9 @@ export {
 } from "./core/tools/index.js";
 // Main entry point
 export { main } from "./main.js";
-// UI components for hooks
+// UI components for extensions
 export { BorderedLoader } from "./modes/interactive/components/bordered-loader.js";
-// Theme utilities for custom tools and hooks
+// Theme utilities for custom tools and extensions
 export {
 	getMarkdownTheme,
 	getSelectListTheme,