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/index.ts

@@ -13,3 +13,13 @@ export {
 	type SessionStats,
 } from "./agent-session.js";
 export { type BashExecutorOptions, type BashResult, executeBash } from "./bash-executor.js";
+export {
+	type HookAPI,
+	type HookError,
+	type HookEvent,
+	type HookEventContext,
+	type HookFactory,
+	HookRunner,
+	type HookUIContext,
+	loadHooks,
+} from "./hooks/index.js";