docs: add pip version-pinning installation instructions for pmll-memory-mcp

[Copilot]

The installation docs only covered npm/npx; there were no pip instructions despite pmll-memory-mcp being published to PyPI as a Python package.

Changes

• README.md / mcp/README.md — Added pip installation block covering latest, pinned versions, and version-switching workflow:

  pip install pmll-memory-mcp           # latest
  
  # Pin to a specific version (choose one)
  pip install pmll-memory-mcp==0.1.0
  # OR
  pip install pmll-memory-mcp==0.2.0
  
  # Switch versions
  pip uninstall -y pmll-memory-mcp
  pip install pmll-memory-mcp==0.1.0

  Includes a callout clarifying that running both version installs leaves you on the later one.

• server.json — Added a pypi entry to packages[] alongside the existing npm entry so the MCP Registry reflects both distribution channels.

Original prompt

To install a specific version, run one command at a time:

pip install pmll-memory-mcp==0.1.0
pip install pmll-memory-mcp==0.2.0

Notes:

• If you run both, the second command will upgrade/replace 0.1.0 with 0.2.0 (so you’ll end up on 0.2.0).
• To switch versions, explicitly install the version you want (or uninstall first):
  pip uninstall -y pmll-memory-mcp
  pip install pmll-memory-mcp==0.1.0

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