Add packages/core: thin SDK wrapper

[Aleexc12]

Issue for this PR

Closes N/A — foundational package, no prior issue.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds packages/core — a minimal wrapper around the OpenCode SDK that spawns and manages the server as a child process, exposing the raw SDK client for consumers.

3 files, ~100 lines total:

• RalphCore.create() — spawns the opencode server, returns a ready-to-use instance
• core.client — raw OpencodeClient from @opencode-ai/sdk, no abstraction layer
• core.dispose() — shuts down the managed server process

Architecture decisions

• Wrap, don't run alongside — Ralph spawns and manages the OpenCode server as a child process. One process to start, one API to consume.
• Thin wrapper, no indirection — The original version had 15 service classes wrapping 57 endpoints with Hono HTTP routes. That was unnecessary — the SDK client already provides a clean, typed API. Core's job is lifecycle management (spawn, connect, dispose), not re-wrapping every endpoint.
• Raw SDK client — Consumers (TUI, future CLI) import @ralph/core, call RalphCore.create(), and use core.client directly with the SDK's native interface. No custom error types, no service layer, no HTTP routes.

How did you verify your code works?

• 24 endpoint tests covering session CRUD, prompting (big-pickle), project/config/providers, files/search, app metadata (agents/skills/commands), path, VCS, and cleanup
• All 24 passing on both Windows (MSYS) and Linux (WSL2)
• Known upstream issue: dispose() doesn't reliably kill the server on Windows (proc.kill() / SIGTERM). Works correctly on Linux.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR