feat(cli): init command for adding Stackwright to existing projects

[perasperaactual]

Parent Issue

Tracked by #177

Problem

scaffold creates a full project from scratch. There's no lighter command for adding Stackwright to an existing Next.js project.

Required Behavior

stackwright init          # in existing project dir
stackwright init my-app   # specify directory

• Generates stackwright.yml if missing
• Creates pages/ directory with root page if missing
• Does NOT overwrite existing files
• Idempotent — running twice is safe

Files to Create

• packages/cli/src/commands/init.ts

Files to Modify

• packages/cli/src/cli.ts — register command
• packages/cli/src/index.ts — export function

Acceptance Criteria

• Init in empty dir creates stackwright.yml and root page
• Init in dir with existing stackwright.yml is no-op (with message)
• Init in dir with existing package.json preserves it
• Works with --json and --no-interactive