feat: scaffold modern Khalti SDK API with verification resources, doc…

[sudiptpa]

PR Highlights

• Complete rebuild of sudiptpa/khalti-sdk-php as a modern, framework-agnostic SDK.
• Industry-style resource API designed for readability and safety:
  • payments()->create(...)
  • payments()->status(...)
  • verification()->parseReturnQuery(...)
  • verification()->verify(...)
  • legacyPayments()->verify(...) / legacyPayments()->status(...)
  • transactions()->all(...) / transactions()->find(...)
• Strongly typed package design:
  • typed models for request/response payloads
  • enums for environment and payment status
  • typed exception hierarchy for precise error handling
• Transport architecture built for extensibility:
  • TransportInterface contract
  • default CurlTransport (no heavy HTTP dependency lock-in)
  • custom transport support for app/framework integrations
• Safe API client core:
  • structured JSON request/response handling
  • authentication header management (Authorization: Key ...)
  • consistent error mapping for auth/validation/api/transport/response failures
• Security-focused payment verification flow:
  • explicit server-side verification using pidx status lookup
  • callback/return parsing and verification decision model
  • amount mismatch protection in verification path
• Explicit documentation that Khalti ePayment does not provide checkout webhook in this flow:
  • backend manual verification is mandatory before fulfillment
  • clear “do not trust return query alone” guidance
• Laravel-focused DX without adding framework dependency in core:
  • Laravel v12/v13 integration guide
  • optional LaravelTransport example in README
• Developer and contributor docs added:
  • complete README from quick-start to advanced usage
  • ARCHITECTURE.md with lifecycle, project map, extension points
• Quality tooling and CI setup from day one:
  • PHPUnit tests
  • PHPStan static analysis
  • PHP lint script
  • GitHub Actions CI matrix for PHP 8.2, 8.3, 8.4, 8.5
• Validation status:
  • tests passing
  • static analysis passing
  • syntax lint passing