[repo] Adopt uv workspaces for extralit-server and extralit (SDK)

[JonnyTran]

Problem

The two Python packages (extralit-server, extralit) have independent pyproject.toml and lockfiles. Shared dev dependencies (ruff, pytest, etc.) drift; cross-package work requires installing each separately.

Approach

1. Add a root pyproject.toml with [tool.uv.workspace] listing both packages.
2. Move shared dev dependencies (ruff, mypy, pytest plugins) to a workspace-level dev group.
3. Single root uv.lock.
4. Update CONTRIBUTING and AGENTS.md so uv sync at the repo root sets up both packages.
5. Update CI workflows (extralit.yml, extralit-server.yml) to use the workspace install.

Acceptance criteria

• uv sync from repo root installs both packages in editable mode.
• A single uv.lock exists at root; per-package lockfiles removed.
• All existing CI jobs pass.
• Docs reflect the new workflow.