Run final Ubuntu acceptance pass for v1.5.0

[codeforester]

Goal

Capture Ubuntu manual bootstrap notes and run a final v1.5.0 acceptance pass on a fresh Ubuntu environment.

Part of #562.

Scope

• Document the manual Ubuntu bootstrap commands used for the first supported runtime path.
• Cover Ubuntu 24.04 in Parallels/ARM64 and GitHub Actions Ubuntu runner assumptions where they differ.
• Keep the repo-location guidance explicit: use native ~/work paths, not mounted host filesystem paths.
• Run and document the final acceptance commands:
  • ./bin/basectl ci check base --format text
  • ./bin/basectl check base --format text
  • ./bin/basectl doctor base --format text
  • env -u BASE_HOME ./bin/base-test

Acceptance Criteria

• docs/linux-support.md or an adjacent setup/testing doc contains the manual Ubuntu bootstrap path.
• The final acceptance commands pass on a fresh or reset Ubuntu environment with documented prerequisites.
• Remaining non-goals, such as apt-backed setup automation and broader distro support, are documented as future work.
• Implement first supported Linux runtime target #562 can be closed or reduced to clearly named follow-up issues after this pass.