Make project Brewfile and uv delegates platform-aware on Ubuntu

[codeforester]

Problem

Ubuntu dogfooding with bankbuddy exposed that project-level delegates are
still macOS/Homebrew-shaped even when Base's own Ubuntu prerequisite path is
healthy.

bankbuddy/base_manifest.yaml declares a uv-managed Python project:

brewfile: Brewfile
python:
  manager: uv

and its Brewfile only declares:

brew "uv"

On Ubuntu, this fails:

basectl setup bankbuddy

with:

Homebrew is required to install Brewfile dependencies from '/home/parallels/work/bankbuddy/Brewfile'.

Installing Homebrew on Ubuntu just to satisfy a project Brewfile would hide
the real support gap. Base needs a platform-aware delegate policy for project
setup.

Desired Behavior

• On macOS, brewfile: Brewfile should continue to delegate to Homebrew.
• On Ubuntu/Debian, a project Brewfile should not be treated as an installable
  prerequisite unless Base explicitly supports that platform behavior.
• For python.manager: uv, Base should provide a Linux-friendly readiness path
  for uv instead of requiring a project Brewfile.
• basectl setup/check/doctor bankbuddy should report clear Ubuntu guidance:
  install or make available uv, then rerun setup/check.
• The design should keep project-owned dependency declarations distinct from
  Base's global Ubuntu apt prerequisites.

Acceptance Criteria

• Add tests for a uv-managed project with brewfile: Brewfile on
  BASE_PLATFORM=linux-debian.
• Define whether Ubuntu setup skips Brewfile delegates, reports them as
  unsupported, or maps known project prerequisites such as uv through a
  platform-aware delegate.
• Update Linux support and project setup docs with the chosen policy.
• Validate with a real bankbuddy Ubuntu smoke once the implementation lands.