Suppress pip self-upgrade notices during Base-managed installs

[codeforester]

Context

During basectl setup bankbuddy, pip emitted its standard self-upgrade notice while Base was installing packages into the Base-managed virtual environment:

[notice] A new release of pip is available: 26.1 -> 26.1.2
[notice] To update, run: /Users/rameshhp/.base.d/base/.venv/bin/python -m pip install --upgrade pip

That notice is not actionable for most Base users during setup. Base owns this runtime, and the message adds noise to otherwise useful setup output.

Desired behavior

Suppress pip self-version notices for Base-owned pip invocations while preserving real pip install output and failures.

Scope

• Add --disable-pip-version-check or equivalent environment handling to Base-managed pip installs.
• Cover Base bootstrap package installs from the shell setup path.
• Cover Python artifact installs from the Python setup layer.
• Do not automatically upgrade pip as part of ordinary setup.
• Do not suppress output from user/project commands that Base merely delegates to.

Acceptance criteria

• basectl setup no longer prints pip self-upgrade notices from Base-managed pip installs.
• Actual pip installation failures still surface clearly with stdout/stderr context.
• Tests cover the generated pip install commands or environment so this behavior stays intentional.