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/index.ts

@@ -0,0 +1,17 @@
+export { type LoadedHook, type LoadHooksResult, loadHooks } from "./loader.js";
+export { type HookErrorListener, HookRunner } from "./runner.js";
+export type {
+	AgentEndEvent,
+	AgentStartEvent,
+	BranchEvent,
+	BranchEventResult,
+	ExecResult,
+	HookAPI,
+	HookError,
+	HookEvent,
+	HookEventContext,
+	HookFactory,
+	HookUIContext,
+	TurnEndEvent,
+	TurnStartEvent,
+} from "./types.js";