Implement first supported Linux runtime target

[codeforester]

Goal

Implement the first supported Linux runtime target for Base after the 1.0 release, starting with an Ubuntu/Debian-family environment.

This issue supersedes the completed planning issue #125. The existing design reference is docs/linux-support.md.

Scope

• Detect Linux platforms through /etc/os-release and classify unsupported distributions clearly.
• Make Base runtime commands work on the selected Linux target when prerequisites already exist.
• Keep the initial support contract runtime-first; do not require Base to install every Linux prerequisite in the first slice.
• Make basectl check, basectl doctor, and basectl ci report Linux prerequisite status without requiring Homebrew or Xcode.
• Preserve macOS behavior and avoid weakening the current Homebrew-based support contract.
• Add CI coverage for the selected Linux target.

Non-goals

• Do not support every Linux distribution in the first pass.
• Do not add Windows or WSL-specific support here.
• Do not introduce a second manifest format for Linux.
• Do not install GUI IDEs on Linux until a real project needs that path.

Acceptance criteria

• The supported Linux target is named and documented.
• Base commands have explicit, tested Linux behavior instead of falling through macOS assumptions.
• basectl check and basectl doctor provide useful Linux findings in text and JSON modes.
• basectl ci can run on Linux with preinstalled prerequisites.
• GitHub Actions validates the Linux runtime path.
• Documentation explains the Linux support boundary and points to follow-up work for full bootstrap support.