feat(examples): make example extensions installable via pi install

[tshu-w]

Summary

Add a pi manifest to the root package.json so that all example extensions can be installed with:

pi install git:github.com/badlogic/pi-mono

Motivation

Currently there's no way to install the example extensions via pi install. Users must manually copy individual files or use pi -e ./path.ts one at a time.

How it works

The root package.json gets pi.extensions: ["./packages/coding-agent/examples/extensions"]. Since extensions/ has no package.json of its own, pi's auto-discovery kicks in and finds:

• All 55 top-level .ts extension files
• Subdirectory extensions without deps (doom-overlay/, dynamic-resources/, plan-mode/, subagent/) via index.ts
• Subdirectory extensions with deps (with-deps/, custom-provider-*/, sandbox/) via their own pi manifests

Changes

• package.json (root) — add pi manifest with pi-package keyword
• packages/coding-agent/examples/extensions/README.md — update usage section with pi install instructions

[github-actions]

Hi @tshu-w, thanks for your interest in contributing!

We ask new contributors to open an issue first before submitting a PR. This helps us discuss the approach and avoid wasted effort.

Next steps:

1. Open an issue describing what you want to change and why (keep it concise, write in your human voice, AI slop will be closed)
2. Once a maintainer approves with lgtm, you'll be added to the approved contributors list
3. Then you can submit your PR

This PR will be closed automatically. See https://github.com/badlogic/pi-mono/blob/main/CONTRIBUTING.md for more details.