Remove dead INSTALL_PYTHON build arg

Python 3 has been unconditionally present since the Debian trixie
upgrade (e58962a, Apr 13) — python3 3.13 ships as a transitive
dependency of the trixie base image. python3-pip (e1029bb) and
python3-venv (3a7ec45) were later added to the base layer on Apr 23
so Mason could install Python-based LSPs (ruff, ansible-lint) into
venvs on nvim startup. MemPalace's pip install (b9c08c3) just
piggybacks on what was already there.

In other words, INSTALL_PYTHON=true has been a no-op reinstall of
already-installed packages for two weeks before MemPalace existed.
The flag is dead weight and the docs that advertise it as meaningful
are misleading. Remove it everywhere.

Users who want Python tooling should use the pre-installed uv/uvx.

.gitignore

@@ -9,3 +9,7 @@
 
 # Personal cloud-init overrides (not shared)
 deploy/my-cloud-init.yml
+
+# MemPalace per-project files (issue #185)
+mempalace.yaml
+entities.json