Migrate pyproject.toml to PEP 621 [project] for native uv support

[bburda]

Summary

The README documents installing and running the server with uv and uvx, but pyproject.toml still uses the legacy Poetry tables ([tool.poetry]) with no PEP 621 [project] section. uv reads dependencies from [project], so the native uv workflows do not work:

• uv sync resolves nothing (no [project.dependencies]), leaving an empty environment.
• uv run <script> re-syncs to that empty state and fails (works only with --no-sync after a manual uv pip install).
• uvx ros2-medkit-mcp-stdio by package name is not possible.

Current uv usage therefore goes through uv pip install -e . and uvx --from git+..., which work but are not the standard uv entry points.

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Proposed solution

Move the project metadata to PEP 621 [project] while keeping the Poetry build backend (Poetry 2.x reads [project]):

• [tool.poetry.dependencies] -> [project.dependencies], with requires-python.
• [tool.poetry.group.dev.dependencies] -> a dev [project.optional-dependencies] extra (or [dependency-groups]).
• [tool.poetry.scripts] -> [project.scripts] (ros2-medkit-mcp-stdio, ros2-medkit-mcp-http).
• Keep build-backend = "poetry.core.masonry.api".

The GitHub-release client wheel needs the PEP 621 direct-reference form, e.g.
ros2-medkit-client @ https://github.com/selfpatch/ros2_medkit_clients/releases/download/py-v0.5.0/ros2_medkit_client-0.5.0-py3-none-any.whl.

After this, uv sync, uv run, and uvx --from git+... <script> all work natively, and the README can drop the uv pip install / --no-sync caveats.

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Additional context

• Must keep poetry install / poetry run working (CI uses Poetry). Poetry 2.x supports [project], so verify on the CI Poetry version and regenerate poetry.lock.
• Verify the client URL dependency still resolves under both uv and Poetry.
• Follow-up to the README uv/uvx documentation.