feat: reset app on Better Auth v3 foundation

[Che-Zhu]

Summary

• reset the application onto the Fulling v3 dedicated-workspace foundation
• replace the previous authentication boundary with GitHub-only Better Auth backed by PostgreSQL
• add per-user kubeconfig persistence, validation, and a user-scoped Kubernetes client boundary
• replace the previous project, sandbox, terminal, and runtime surfaces with a minimal protected workspace entry
• add the v3 baseline migration, CI coverage, browser tests, and deployment/reset documentation

Why

The previous product model no longer matched the v3 architecture. This change establishes a clean identity and Kubernetes credential foundation so Workspace domain work can proceed without carrying compatibility code from v2.

Impact

This is an intentional reset, not a backwards-compatible migration. Existing v2 database records are not migrated, and Kubernetes resources created by v2 must be inventoried before deployment. The user-level kubeconfig boundary is foundational and is not the final Workspace Runtime ownership model.

Validation

• corepack pnpm lint
• corepack pnpm test — 30 tests passed
• corepack pnpm prisma:validate
• corepack pnpm test:e2e — 18 tests passed across desktop, mobile, and narrow viewports
• corepack pnpm build

Closes #176