Add unified extensions system (not wired up yet)

New src/core/extensions/ directory with:
- types.ts: merged types from hooks and custom-tools
- loader.ts: single loader for extensions
- runner.ts: ExtensionRunner for event emission
- wrapper.ts: tool wrapping utilities
- index.ts: exports

Key changes from old system:
- Single ExtensionAPI with registerTool() for LLM-callable tools
- Tools use ExtensionContext (has UI access)
- No onSession callback on tools (use pi.on events instead)

refs #454

packages/coding-agent/src/core/extensions/index.ts

@@ -0,0 +1,100 @@
+/**
+ * Extension system - unified hooks and custom tools.
+ */
+
+export { discoverAndLoadExtensions, loadExtensions } from "./loader.js";
+export type { BranchHandler, ExtensionErrorListener, NavigateTreeHandler, NewSessionHandler } from "./runner.js";
+export { ExtensionRunner } from "./runner.js";
+export type {
+	AgentEndEvent,
+	AgentStartEvent,
+	// Re-exports
+	AgentToolResult,
+	AgentToolUpdateCallback,
+	AppendEntryHandler,
+	BashToolResultEvent,
+	BeforeAgentStartEvent,
+	BeforeAgentStartEventResult,
+	// Events - Agent
+	ContextEvent,
+	// Event Results
+	ContextEventResult,
+	CustomToolResultEvent,
+	EditToolResultEvent,
+	ExecOptions,
+	ExecResult,
+	// API
+	ExtensionAPI,
+	ExtensionCommandContext,
+	// Context
+	ExtensionContext,
+	// Errors
+	ExtensionError,
+	ExtensionEvent,
+	ExtensionFactory,
+	ExtensionFlag,
+	ExtensionHandler,
+	ExtensionShortcut,
+	ExtensionUIContext,
+	FindToolResultEvent,
+	GetActiveToolsHandler,
+	GetAllToolsHandler,
+	GrepToolResultEvent,
+	LoadExtensionsResult,
+	// Loaded Extension
+	LoadedExtension,
+	LsToolResultEvent,
+	// Message Rendering
+	MessageRenderer,
+	MessageRenderOptions,
+	ReadToolResultEvent,
+	// Commands
+	RegisteredCommand,
+	RegisteredTool,
+	SendMessageHandler,
+	SessionBeforeBranchEvent,
+	SessionBeforeBranchResult,
+	SessionBeforeCompactEvent,
+	SessionBeforeCompactResult,
+	SessionBeforeSwitchEvent,
+	SessionBeforeSwitchResult,
+	SessionBeforeTreeEvent,
+	SessionBeforeTreeResult,
+	SessionBranchEvent,
+	SessionCompactEvent,
+	SessionEvent,
+	SessionShutdownEvent,
+	// Events - Session
+	SessionStartEvent,
+	SessionSwitchEvent,
+	SessionTreeEvent,
+	SetActiveToolsHandler,
+	// Events - Tool
+	ToolCallEvent,
+	ToolCallEventResult,
+	// Tools
+	ToolDefinition,
+	ToolRenderResultOptions,
+	ToolResultEvent,
+	ToolResultEventResult,
+	TreePreparation,
+	TurnEndEvent,
+	TurnStartEvent,
+	WriteToolResultEvent,
+} from "./types.js";
+// Type guards
+export {
+	isBashToolResult,
+	isEditToolResult,
+	isFindToolResult,
+	isGrepToolResult,
+	isLsToolResult,
+	isReadToolResult,
+	isWriteToolResult,
+} from "./types.js";
+export {
+	wrapRegisteredTool,
+	wrapRegisteredTools,
+	wrapToolsWithExtensions,
+	wrapToolWithExtensions,
+} from "./wrapper.js";