docs(readme): recommend pipx for install (fixes PEP 668 + global dep conflicts)

[OmkarKirpan]

Summary

The pip install mempalace line in the README either fails on
PEP 668-managed Pythons (Debian/Ubuntu, Homebrew) or silently upgrades
chromadb / numpy / grpcio / click in the user's global
site-packages and breaks unrelated tools.

A real run on macOS today, against a Python that already had numba,
litellm, tutor, and the OpenTelemetry instrumentation packages
installed, produced this from a single pip install mempalace:

opentelemetry-instrumentation 0.62b0 requires
  opentelemetry-semantic-conventions==0.62b0, but you have 0.62b1
numba 0.61.2 requires numpy<2.3,>=1.24, but you have numpy 2.4.4
litellm 1.80.11 requires grpcio<1.68.0,>=1.62.3, but you have grpcio 1.80.0
tutor 20.0.2 requires click<8.3.0,>=8.0, but you have click 8.3.3

mempalace ships a CLI, so pipx install mempalace (or
uv tool install mempalace) is the right default — it isolates the
install and still puts mempalace on PATH. Plain pip install is
kept as the documented alternative for users who want import mempalace inside their own activated venv.

Refs #284 (PEP 668 / --break-system-packages).

What changed

• README.md "Install" section: lead with pipx install mempalace,
  mention uv tool install as an equivalent, briefly explain why
  (PEP 668 + global dep conflicts), and keep pip install for the
  in-venv case.
• No code changes.

Test plan

• git diff README.md — only the Install section changes
• pipx install mempalace succeeds on a system Python where
  pip install mempalace would either trip PEP 668 or
  downgrade/upgrade global packages

[igorls]

Looks good and it's the way it should be installed indeed. @bensig @milla-jovovich ok on this ?