Consume expanded GitHub Bash helpers in Base

[codeforester]

Problem

After base-bash-libs exposes the next set of generic GitHub helpers, Base should consume them instead of continuing to carry duplicate generic plumbing in:

• cli/bash/commands/basectl/subcommands/gh.sh
• cli/bash/commands/basectl/subcommands/repo.sh

The previous Base issue consumed the first thin helper layer. The current follow-up is for the remaining generic repo/default-branch/API/worktree primitives found during the Bash review.

Dependency

Depends on basefoundry/base-bash-libs#119.

Scope

• Import and use the expanded gh/lib_gh.sh helpers through import_base_lib.
• Replace only generic helper code in gh.sh and repo.sh.
• Keep Base product workflow in Base: issue defaults, branch naming, Project metadata, repo baseline generation, and generated PR bodies.
• Preserve existing command output and exit behavior.

Acceptance Criteria

• Focused basectl gh and basectl repo tests cover the migrated paths.
• No Base-specific policy moves into base-bash-libs.
• Shell syntax, ShellCheck, and relevant Base test gates pass.