test: keep .env out of the suite; report the real interpreter in notebooks

[jqnatividad]

Two related bits of drift, both found the hard way while setting up a local instance.

1. The suite was not actually hermetic

CONTRIBUTING.md, the CI comment and the suggested_commands memory all stated
the suite needs no .env. True — but core/config.py reads one for every
field and builds its settings singleton at import time, so a developer's local
.env silently changed what the suite asserted.

Concretely: following the README's own troubleshooting advice and setting
NOTEBOOK_VERIFICATION_ENABLED=false turned the suite red. That is how this was
found.

• tests/conftest.py sets DATA_CONCIERGE_ENV_FILE="" before anything imports
  config; config.py honours it, empty meaning no env file.
• Three tests asserted a shipped default (or a security guard's default posture)
  by reading the live singleton, which also reflects the process environment.
  They now assert Settings.model_fields[...].default, or pin the value with
  monkeypatch, so an exported variable cannot reach them either:
  test_ships_enabled, test_ships_locked_down,
  test_model_validator_rejects_on_update_construction.
• The three texts now describe the real guarantee, including the part the
  conftest guard does not cover.

2. Generated notebooks claimed an impossible interpreter

notebook_generator.py hardcoded language_info.version = "3.11" in two places.
That has been wrong since requires-python became >=3.12. The notebook is the
deliverable — reproducible, citable, independently verifiable — so its provenance
has to be true. Both sites now report platform.python_version().

Verified

• 802 passed with a hostile .env (both flags flipped) and the same two
  variables exported in the shell
• 802 passed clean; ruff check clean
• A generated notebook confirmed to carry the running interpreter
  (language_info.version == platform.python_version())

Note

MCP_ALLOW_PRIVATE_URLS was the second latent instance of this bug — the
variable exists precisely so local development can point at localhost MCP
servers, which makes the developer most likely to run the suite the one most
likely to have it set.

🤖 Generated with Claude Code