[codex] Add Swift SDK feature parity

[devteapot]

Summary

Adds a Swift Package Manager SDK under packages/swift/slop-ai with core protocol, provider, consumer, discovery, bridge, and Apple App Intents support.

This PR is now intentionally limited to the Swift SDK. Cross-SDK protocol refinements and desktop/extension security hardening were extracted into a stacked follow-up PR.

What changed

• Added Swift wire models, descriptor normalization, tree assembly, diffing, scaling, state mirror, tools, store helpers, consumer APIs, and provider APIs.
• Added provider transports for stdio NDJSON, Darwin Unix sockets, and SwiftNIO WebSocket server support.
• Added consumer transports for URLSession WebSocket and Darwin Unix sockets.
• Added bridge discovery and relay transport support for browser-backed providers.
• Hardened local provider discovery and Unix socket handling.
• Made SlopServer concurrency-safe with lock-backed state mutation and out-of-lock delivery.
• Added Apple App Intents entity/action projection.
• Added focused parity and robustness tests.

Scope

The diff against main contains only:

• packages/swift/slop-ai/**
• the Swift-related .gitignore entry

Validation

• cd packages/swift/slop-ai && swift test
• 77 tests passed