docs: update extension examples READMEs and fix plan-mode references
- Add missing extensions to examples/extensions/README.md:
questionnaire.ts, truncated-tool.ts, model-status.ts, rainbow-editor.ts,
custom-footer.ts, custom-header.ts, overlay-test.ts, shutdown-command.ts,
interactive-shell.ts, claude-rules.ts, mac-system-theme.ts
- Update plan-mode.ts -> plan-mode/ (now a directory)
- Add new System Integration section
- Update docs/extensions.md plan-mode.ts -> plan-mode/index.ts (17 refs)
- Add questionnaire.ts to registerTool and ctx.ui.select() examples
- Expand examples/README.md description