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/11-sessions.ts

@@ -4,7 +4,7 @@
  * Control session persistence: in-memory, new file, continue, or open specific.
  */
 
-import { createAgentSession, SessionManager } from "../../src/index.js";
+import { createAgentSession, SessionManager } from "@mariozechner/pi-coding-agent";
 
 // In-memory (no persistence)
 const { session: inMemory } = await createAgentSession({