Expand reusable GitHub Bash helpers

[codeforester]

Problem

Base still owns generic GitHub/Git helper behavior in cli/bash/commands/basectl/subcommands/gh.sh and cli/bash/commands/basectl/subcommands/repo.sh that is reusable outside Base:

• GitHub repository inference from origin remotes.
• Default branch discovery.
• Basic gh api/command diagnostics and bounded retry behavior for transient API pressure.
• Worktree/branch parsing primitives that are not tied to Base issue categories or Project policy.

base-bash-libs already has lib/bash/gh/lib_gh.sh, but it currently covers only thin gh availability, auth diagnostics, failure reporting, and command execution.

Scope

• Extend base-bash-libs gh/lib_gh.sh with small generic primitives proven by the remaining Base call sites.
• Keep Base-specific concepts out of the shared library: issue categories, Project metadata, branch naming policy, repo baselines, generated PR bodies, and worktree naming conventions.
• Add BATS coverage with fake git/gh commands where useful.
• Document the expanded reusable surface in lib/bash/gh/README.md.

Acceptance Criteria

• New helper names and contracts are documented.
• Tests cover success and failure cases for the extracted helpers.
• The shared library remains dependency-free apart from the stdlib guard.
• A follow-up Base issue can consume the helpers without changing Base workflow semantics.

[codeforester]

Companion Base consumer issue: basefoundry/base#1488.

That issue should wait for this shared helper expansion, then replace only generic helper code in Base while keeping Base-specific workflow policy in Base.