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/08-slash-commands.ts

@@ -4,7 +4,12 @@
  * File-based commands that inject content when invoked with /commandname.
  */
 
-import { createAgentSession, discoverSlashCommands, type FileSlashCommand, SessionManager } from "../../src/index.js";
+import {
+	createAgentSession,
+	discoverSlashCommands,
+	type FileSlashCommand,
+	SessionManager,
+} from "@mariozechner/pi-coding-agent";
 
 // Discover commands from cwd/.pi/commands/ and ~/.pi/agent/commands/
 const discovered = discoverSlashCommands();