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/examples/sdk/09-api-keys-and-oauth.ts

@@ -11,7 +11,7 @@ import {
 	discoverModels,
 	ModelRegistry,
 	SessionManager,
-} from "../../src/index.js";
+} from "@mariozechner/pi-coding-agent";
 
 // Default: discoverAuthStorage() uses ~/.pi/agent/auth.json
 // discoverModels() loads built-in + custom models from ~/.pi/agent/models.json