chore: migrate development workflows to uv and pin mock server tooling

[mcgrew-oai]

Summary

This updates the repository's development and CI workflows to use uv as the primary Python workflow tool and replaces the ad hoc npm exec mock-server install path with a pinned pnpm-managed dependency.

Changes

• migrate dev dependencies from Rye config to dependency-groups
• add uv.lock as the primary Python lockfile
• keep requirements.lock and requirements-dev.lock as compatibility exports
• update bootstrap, lint, format, test, and publish scripts to use uv
• update CI workflows to use pinned astral-sh/setup-uv
• update devcontainer and contributor docs to use uv
• add a pinned pnpm workspace for @stdy/cli
• update mock/test flows to use corepack pnpm exec steady
• add .node-version and pnpm-lock.yaml

Why

The main driver for this change is reducing supply-chain risk in the repository's package-management paths.

In particular, this change:

• standardizes Python workflow management on uv with an explicit committed lockfile
• removes deprecated Rye-based repo workflow configuration as an active install path
• avoids fresh runtime resolution of the mock server tool during CI by pinning it behind a committed pnpm lockfile
• makes dependency installation paths more explicit and reviewable in CI and local development

Verification

• ./scripts/bootstrap
• ./scripts/lint
• uv lock --check
• uv build
• ./scripts/mock --daemon
• ./scripts/test tests/test_client.py -q
• git diff --check

Notes

• requirements.lock and requirements-dev.lock are still kept for compatibility with existing tooling paths.
• no runtime SDK behavior is intended to change.