Make init templates uv-native for Nitro cloud

[manish-wekan]

Summary

• Init templates (starter, pizzaz, flight-booking) now ship PEP 621 pyproject.toml, .python-version, and uv.toml. init rewrites the project name and runs uv lock when uv is on PATH.
• nitrostack-py install uses uv sync for cloud-shaped apps, and still uses pip install -r requirements.txt when nitrostack is pinned as a local/editable path so unpublished SDK testing keeps working.
• The SDK repo itself now has .python-version, uv.toml, and a committed uv.lock (resolves mcp>=2,<3). Template locks are not checked in, so they do not pin PyPI 0.3.4 / mcp>=1.

Closes #28
Closes #29

Test plan

• nitrostack-py init demo --template python-starter writes pyproject.toml, .python-version, uv.toml; name matches the folder
• With uv on PATH, init creates uv.lock in the new project
• Without uv, init still succeeds and prints the skip/lock hint
• nitrostack-py install in a generated app runs uv sync when requirements.txt is just nitrostack
• Put -e /path/to/nitrostack-python-sdk in requirements.txt and confirm install uses pip, not uv sync
• pytest tests/test_cli.py tests/test_cli_install.py