Update docs for ctx.ui.editor() and handoff example

- Added ctx.ui.editor() to hooks.md and custom-tools.md
- Added ctx.ui.editor() to CHANGELOG.md
- Added handoff.ts to examples/hooks/README.md

packages/coding-agent/examples/hooks/README.md

@@ -27,6 +27,7 @@ cp permission-gate.ts ~/.pi/agent/hooks/
 | `qna.ts` | Extracts questions from last response into editor via `ctx.ui.setEditorText()` |
 | `snake.ts` | Snake game with custom UI, keyboard handling, and session persistence |
 | `status-line.ts` | Shows turn progress in footer via `ctx.ui.setStatus()` with themed colors |
+| `handoff.ts` | Transfer context to a new focused session via `/handoff <goal>` |
 
 ## Writing Hooks