Make SDKs publishable, harden API, and add CI/release workflows (alpha)

[thomasdreyer]

Motivation

• Provide a publishable React Native SDK and seed a .NET mobile client so mobile teams (JS/.NET) can consume OpenSync without breaking API contracts.
• Stabilize server-side validation and authentication to reduce breaking changes for package consumers and improve security posture.
• Add automated package validation and release scaffolding (Changesets + GitHub Actions) to enable repeatable alpha releases.

Description

• Added a publishable npm package @opensync/sdk-react-native with package.json, TypeScript tsconfig.*, build scripts, and a small fetch-based client implementation to avoid runtime axios friction.
• Seeded a .NET SDK package OpenSync.Client with csproj, client implementation, readme, and NuGet metadata targeting net8.0;net8.0-android;net8.0-ios.
• Hardened API: added global ValidationPipe in apps/src/main.ts, DTOs for auth/sync (AuthDto, PushDto, PullDto), JWT Strategy + JwtAuthGuard, exported PassportModule, safer auth exceptions, and fixed pull to use TypeORM's MoreThan(...) with deterministic ordering.
• Repo/tooling: added Changesets config and pre-mode for alpha, updated root package.json scripts/devDeps and packageManager, migrated turbo.json pipeline→tasks, added tsconfig.api.json, and aligned apps/api build scripts for monorepo layout.
• CI & release: added .github/workflows/api.yml (API CI), packages.yml (pack & dotnet pack checks), and release.yml (manual release that publishes npm alpha and builds NuGet artifacts).
• Docs: rewrote README.md and added/updated docs/* with real install, build, publish, and roadmap guidance (npm, NuGet, native SDK roadmap).

Testing

• Ran npm run build at repo root and the Turborepo build (succeeded). ✅
• Built the React Native package with npm --workspace @opensync/sdk-react-native run build (succeeded). ✅
• Verified npm package contents via npm --workspace @opensync/sdk-react-native pack --dry-run (succeeded and inspected tarball contents). ✅
• Built the API package with npm --workspace opensync-api run build (TypeScript compile succeeded). ✅
• npm ci / npm install failed in this environment due to missing lockfile and registry policy (403), so dependency install via CI requires a normal environment with registry access or a lockfile. ⚠️
• dotnet pack packages/sdk-dotnet/OpenSync.Client/OpenSync.Client.csproj -c Release failed here because dotnet is not installed in the execution environment, but the csproj and code were validated statically. ⚠️

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Codex Task