Add Pi package output

[chubes4]

Summary

• Adds a generated plugins/pi/ package that exposes the shared Build with WordPress skills through Pi's official pi.skills package manifest surface.
• Documents the official Pi surface and compatibility path: skills package now, future TypeScript extension required for direct Studio MCP because Pi does not include built-in MCP support.
• Extends pnpm verify to check the Pi manifest, shared skills, README evidence, and absence of generated MCP/telemetry MCP files.

Official Pi source evidence

• Pi docs identify @earendil-works/pi-coding-agent as the installable coding-agent package and link the official repo: https://pi.dev/docs/latest
• Pi coding-agent package source: https://github.com/earendil-works/pi/tree/main/packages/coding-agent
• Pi package format supports package.json pi manifests and the pi-package keyword: https://pi.dev/docs/latest/packages
• Pi skills support follows Agent Skills and loads skills from package manifests/directories: https://pi.dev/docs/latest/skills
• Pi extensions are TypeScript modules for tools, commands, events, providers, and UI: https://pi.dev/docs/latest/extensions
• Pi README documents the compatibility boundary: "No MCP" and MCP support should be built as an extension if needed: https://github.com/earendil-works/pi/tree/main/packages/coding-agent

Verification

• pnpm build && pnpm verify

Closes #27.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Investigated official Pi documentation/source links, drafted the Pi package generator/docs/verification changes, and ran build/verify. Chris remains responsible for review and merge.