Prepare Base v1.6.0 release

[codeforester]

Prepare the Base v1.6.0 release metadata and release checks now that the Ubuntu support work has landed.

Scope:

• Move the current Unreleased changelog entries into a dated v1.6.0 section.
• Update VERSION, README release references, and AI context current-release metadata.
• Bind Base CI source-checkout jobs to the latest published base-bash-libs release, v1.1.0.
• Run the release-prep validation stack and guarded release check from a clean worktree.
• Publish v1.6.0 after the release-prep PR merges, then hand off the Homebrew tap update.

Acceptance:

• VERSION reports 1.6.0.
• CHANGELOG.md contains a v1.6.0 section with the Ubuntu setup/support work.
• README.md and .ai-context current-release references match 1.6.0.
• Base workflow checkouts pin basefoundry/base-bash-libs to the v1.1.0 release commit.
• env -u BASE_HOME ./bin/base-test passes.
• basectl release check --version 1.6.0 passes from a clean release-prep worktree.