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/01-minimal.ts

@@ -5,7 +5,7 @@
  * from cwd and ~/.pi/agent. Model chosen from settings or first available.
  */
 
-import { createAgentSession } from "../../src/index.js";
+import { createAgentSession } from "@mariozechner/pi-coding-agent";
 
 const { session } = await createAgentSession();