Add Ubuntu apt-backed Base setup validation for v1.6

[codeforester]

Context

Base v1.6.0 is expected to make Ubuntu/Debian setup more than a read-only runtime check: Base can install simple apt prerequisites behind explicit --dry-run / --yes, and the dev profile maps BATS, GitHub CLI, and ShellCheck to apt-backed tools.

base-demo should validate that new Base support path without turning the whole interactive demo into a Linux-supported workflow.

Scope

• Depends on Pin base-demo CI to Base v1.6.0 #159.
• Add or expand an Ubuntu CI path that uses Base v1.6.0.
• Exercise basectl setup --dry-run on Ubuntu and assert the apt plan is visible.
• Exercise the supported non-interactive setup path, likely basectl setup --yes for Base-owned prerequisites.
• Exercise basectl setup --profile dev or a narrow dev-profile check if the Base 1.6 contract supports it cleanly in CI.
• Keep basectl ci check base-demo --manifest ./base_manifest.yaml --format json as the project-level read-only health check.
• Avoid starting services or requiring Docker on Ubuntu in this slice.

Validation

• ./tests/validate.sh
• git diff --check
• GitHub Actions Ubuntu job proves the new setup path.
• Existing macOS validation remains unchanged.

Dependencies

• Pin base-demo CI to Base v1.6.0 #159 must land first.
• Base v1.6.0 must be tagged.

Non-goals

• Do not claim the full interactive base-demo demo is Linux-supported unless a separate PR proves setup, activation, check/doctor, build/test, and demo behavior end to end.
• Do not add Docker or service startup requirements to Ubuntu CI in this issue.