Add support for monorepos

Problem
The wizard currently fails when run from monorepo root directories. When executed from the root of a workspace containing multiple apps, it cannot locate the target framework (in our case Next.js) and throws an error instead of discovering and installing PostHog in the appropriate subpackage.

Current Behavior

• Must be executed from the specific app directory containing the target framework
• Fails with error when run from monorepo root
• No automatic discovery of workspace structure

Expected Behavior

• Should work when run from monorepo root
• Automatically discover workspace packages containing supported frameworks
• Install PostHog in all relevant packages or prompt user to select specific ones

Our project directories look something like this:

project/
├── package.json
├── pnpm-workspace.yaml
├── apps/
│   ├── web/                    # Next.js app
│   │   ├── package.json
│   │   └── src/
│   └── api/                    # Fastify API
│       ├── package.json
│       └── src/
└── packages/
    └── shared/
        ├── package.json
        └── src/

Proposed Solution

1. When framework detection fails in root package.json, check for workspace indicators:
   • pnpm-workspace.yaml
   • "workspaces" field in root package.json
2. Parse workspace configuration to discover all packages
3. Scan each package for supported frameworks (Next.js, React, etc.)
4. Present options to user:
   • Install in all detected compatible packages
   • Select specific packages for installation
   • Skip packages that already have PostHog configured

Start at the wizard's framework-detection entry point and inspect how it reads the root package.json. Then review the proposed workspace indicators in pnpm-workspace.yaml and package.json, and trace how detected packages are presented for installation. Done means the wizard can discover supported framework packages from a monorepo root and install in all, selected, or unconfigured packages as described.