previewsmcp init — scaffold project config and setup plugin

[obj-p]

Problem

Setting up PreviewsMCP for a project requires manual boilerplate:

1. Creating a .previewsmcp.json with platform, device, and trait defaults
2. For the setup plugin: creating a PreviewSetup/ package directory, Package.swift with the PreviewsSetupKit dependency, and a PreviewSetup protocol stub
3. Wiring the setup block in .previewsmcp.json

This is error-prone and not discoverable from the CLI alone.

Proposal

Add a previewsmcp init subcommand that scaffolds project configuration interactively or with flags:

# Scaffold .previewsmcp.json with auto-detected defaults
previewsmcp init

# Also generate the setup plugin package
previewsmcp init --setup

# Non-interactive with explicit values
previewsmcp init --platform ios --device "iPhone 16 Pro" --setup

What init generates

Always:

• .previewsmcp.json with sensible defaults (auto-detect platform from project if possible, per Auto-detect platform from project instead of defaulting to macOS #83)

With --setup:

• PreviewSetup/Package.swift with PreviewsSetupKit dependency
• PreviewSetup/Sources/<ModuleName>/Setup.swift with a PreviewSetup protocol stub
• Adds the setup block to .previewsmcp.json

Considerations

• Should not overwrite existing files without --force
• Module name and type name could be prompted interactively or accept --module-name / --type-name flags
• Could detect existing .previewsmcp.json and offer to add missing fields (e.g., add setup to an existing config)
• See docs/setup-plugin.md for the full integration walkthrough