Add setEditorText/getEditorText to hook UI context, improve custom() API

- Add setEditorText() and getEditorText() to HookUIContext for prompt generator pattern
- custom() now accepts async factories for fire-and-forget work
- Add CancellableLoader component to tui package
- Add BorderedLoader component for hooks with cancel UI
- Export HookAPI, HookContext, HookFactory from main package
- Update all examples to import from packages instead of relative paths
- Update hooks.md and custom-tools.md documentation

fixes #350

packages/coding-agent/src/core/sdk.ts

@@ -140,7 +140,7 @@ export interface CreateAgentSessionResult {
 // Re-exports
 
 export type { CustomTool } from "./custom-tools/types.js";
-export type { HookAPI, HookFactory } from "./hooks/types.js";
+export type { HookAPI, HookContext, HookFactory } from "./hooks/types.js";
 export type { Settings, SkillsSettings } from "./settings-manager.js";
 export type { Skill } from "./skills.js";
 export type { FileSlashCommand } from "./slash-commands.js";