[P0] Pick a monorepo strategy and add workspace tooling

[Calebux]

Summary

The repo has root, client, backend, sdk, shared, and contracts packages but no workspace manifest such as pnpm-workspace.yaml.

Scope

• Area: architecture
• Priority: P0
• Backlog ID: #2

Acceptance Criteria

• Workspace tool choice is documented.
• Root scripts exist for install, test, lint, and typecheck across packages.
• Package boundaries and ownership are documented.

Implementation Notes

• Keep the implementation aligned with current architecture decisions and security constraints in this repo.
• Include tests for changed behavior and update docs where relevant.

Definition of Done

• Acceptance criteria met
• Tests added/updated and passing
• Documentation updated
• No security regressions introduced

Links

• Backlog source: docs/repo-issue-backlog-2026-05.md