feat: Vercel adapter project path configuration

Summary

Add project path configuration to the Vercel adapter for cases where vercel env pull needs to run from a specific directory.

Context

File: internal/adapters/vercel/vercel.go:81

// TODO: Consider adding project path configuration

Problem

Currently the Vercel adapter runs vercel env pull from the temp directory. Some Vercel projects may need the command run from the actual project directory to detect the linked project.

Implementation

Option 1: ProjectPath field

type VercelAdapter struct {
    binary      string
    projectPath string  // If set, cd to this path before running vercel
}

func NewWithPath(projectPath string) *VercelAdapter

Option 2: Use vercel --cwd flag

vercel env pull --cwd /path/to/project --environment production

Update .secrets.json

{
  "source": "vercel",
  "project": "my-app",
  "project_path": "/path/to/project",  // optional
  "scope": "production"
}

Acceptance

• Vercel adapter can run from specified project path
• Works with linked Vercel projects
• Backwards compatible (project_path is optional)

Start at internal/adapters/vercel/vercel.go:81 and inspect how the adapter currently invokes vercel env pull and how .secrets.json fields are parsed. Determine whether the project path should use a field or the --cwd option, then verify the optional setting works for linked projects while existing configurations remain compatible.