feat: manage uv tool plugins

[D3f0]

Summary

• Detect active uv-managed invoke-toolkit installations and report plugin versions.
• Add internal -x plugin.list/add/remove/update tasks for uv tool plugin management.
• Preserve uv receipt requirements, including Git and editable sources, during reinstall.
• Document the uv-only workflow and future package-manager scope.

Verification

• uv run pytest — 742 passed, 18 skipped.
• Focused uv/plugin and create-task tests pass.
• Real invoke-toolkit-litellm Git plugin integration passes.
• Clean ghcr.io/astral-sh/uv:trixie Podman integration passes using an isolated wheel/tool directory.
• Ruff, syntax compilation, and diff checks pass.

Closes #84