feat: migrate OpenCode plugin to v2

[HelgeSverre]

Summary

• migrate the package to @opencode/plugin 2.0.3 and the native v2 Plugin.define entrypoint
• retain the documented OpenCode v1.18.29+ combined server() compatibility path
• register Sema MCP through the v2 MCP transform and inject the bundled guide through the v2 session context hook
• document v2 LSP and formatter limits, update configuration examples, and release the package as 2.0.0

Verification

• bun run format:check
• bun run typecheck
• bun test (27 passing)
• bun run build
• npm pack --dry-run

Closes #1