Honor uv-managed project venvs in setup and check

[codeforester]

Summary

• Make uv-managed projects bypass the historical ~/.base.d/<project>/.venv setup/check gate.
• Run uv-managed project setup/check through Base's own Python runtime while exposing <project-root>/.venv as BASE_PROJECT_VENV_DIR.
• Add BASE-P154 to report missing or ready uv project virtualenvs with uv sync guidance.

Root Cause

setup_common.sh still required the legacy Base-managed project venv before invoking the Python project layer, even for manifests that explicitly declare python.manager: uv.

Validation

• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/check.bats cli/bash/commands/basectl/tests/setup.bats cli/bash/commands/basectl/tests/project-command-helpers.bats
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_setup/tests/test_uv.py -q
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_setup/tests/test_command_lint.py cli/python/base_setup/tests/test_pyproject.py -q
• git diff --check
• env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash ./bin/base-test

Smoke

• basectl setup base-uv-smoke --manifest /private/tmp/base-uv-smoke/base_manifest.yaml --dry-run shows uv sync and does not create ~/.base.d/base-uv-smoke/.venv.
• basectl check base-uv-smoke --manifest /private/tmp/base-uv-smoke/base_manifest.yaml --format json reaches uv diagnostics and reports BASE-P154 with uv sync guidance. The command used a fake writable HOME, so the base prerequisite layer also reported a Homebrew Python formula error unrelated to the uv project check.

Demo Impact

• None.

Fixes #896