Fix Zenbu IDE root detection for Pi house-rules prompt

[dr-baker]

Problem

The app-owned Pi extension that injects Zenbu editing guidance tries to identify the Zenbu source root with:

const ROOT_MARKERS = ["zenbu.config.ts", "AGENTS.md"] as const

That root identification is flawed for two separate reasons:

1. It unnecessarily requires AGENTS.md. The release/dev checkout may not contain AGENTS.md, so root resolution returns null even when the directory is clearly the running app root. That silently prevents the house-rules prompt from being appended.
2. It identifies any Zenbu.js app, not specifically the Zenbu IDE app. zenbu.config.ts is a framework-level marker for any app built with Zenbu.js. The prompt being injected is IDE-specific guidance about editing this app, so the detector should verify that the host app is the Zenbu IDE, e.g. via package.json#name === "zenbu" or another app-specific identity marker.

Expected behavior

The Pi house-rules extension should resolve the running Zenbu IDE app root reliably and only inject IDE-specific instructions when the host app is actually the Zenbu IDE.

Suggested direction

Instead of walking from the session/project cwd and requiring docs files as root markers:

• derive the host app root from the running Zenbu app context, such as process.env.ZENBU_CONFIG_PATH → dirname(configPath);
• read the app package.json and require name === "zenbu" before enabling the IDE-specific prompt;
• treat docs such as AGENTS.md as optional prompt content, not as required root markers.

This keeps framework-level Zenbu.js app detection separate from Zenbu IDE-specific behavior.