Split marketplace definition into two plugins: skills-only + ty LSP-only

## Summary

Proposing splitting the current plugin in two:

1. `astral` (skills only: uv, ruff, ty guidance)
2. `ty-lsp` (LSP only: `ty` language server)

## Problem

Users cannot selectively disable the `ty` LSP if using a different lsp plugin while keeping the skills.

## Proposed change

- Create a skills-only plugin.
- Introduce a separate `ty-lsp` plugin that contains only:
- `lspServers.ty.command = "uvx"`
- `lspServers.ty.args = ["ty@latest", "server"]`
- Python file extensions mapping (`.py`, `.pyi`).
- Publish both plugins in the Astral marketplace.
- Update docs to show independent install paths and migration guidance.

Start by locating the current marketplace plugin definition and its publishing metadata, then review the existing documentation. Separate the skills for uv, ruff, and ty from the ty LSP configuration, publish the two named plugins, and update the documented install and migration paths.

Written by the indexing model from the issue text.