Scrub active project environment for explicit project setup

[codeforester]

Problem

When running from an active Base project shell, basectl setup <other-project>
can inherit active-project environment such as BASE_PROJECT_VENV_DIR and route
the explicit target project to the wrong virtual environment.

During Ubuntu dogfooding, running from the active base shell:

basectl setup base-demo

failed because base-demo was evaluated against the base project venv:

Project virtual environment '/home/parallels/.base.d/base/.venv' uses Python 3.12,
but python.requires_python '3.13' selects Python 3.13.

The immediate manual workaround is to scrub active project variables:

env -u BASE_PROJECT -u BASE_PROJECT_ROOT -u BASE_PROJECT_MANIFEST -u BASE_PROJECT_VENV_DIR \
  basectl setup base-demo --recreate-venv

Base should do the safe thing automatically when the user passes an explicit
project name.

Desired Behavior

• basectl setup <project>, check <project>, and doctor <project> should
  not reuse active-project env from a different project.
• Explicit project setup should route to the explicit project's venv contract.
• Existing override behavior should remain intentional: if users explicitly
  pass an override for the same target project, Base should preserve the
  supported override path.
• Error messages should point at the target project, not the previously active
  shell project.

Acceptance Criteria

• Add regression coverage for running basectl setup other-project while
  BASE_PROJECT, BASE_PROJECT_ROOT, BASE_PROJECT_MANIFEST, and
  BASE_PROJECT_VENV_DIR are set for a different project.
• Confirm the fix covers setup and the read-only check/doctor paths where the
  same route helper is used.
• Validate with focused shell/Python tests and the Base test gate.

[codeforester]

Resolved by PR #1373, merged as 5e5ad42.