Move generic Git worktree and branch helpers out of lib_gh

[codeforester]

Problem

lib/bash/gh/lib_gh.sh is documented as a GitHub CLI library, but several public helpers are generic local Git operations and never invoke gh:

• gh_detect_default_branch
• gh_worktree_path_for_branch
• gh_list_worktree_branches
• gh_branch_upstream
• gh_branch_merged_to_ref
• gh_list_remote_branches

Default-branch detection also duplicates _git_expected_update_branch in lib_git.sh. This blurs library ownership and makes Git-only callers import a GitHub-specific module.

Scope

• Move generic branch, remote, and worktree primitives into lib_git.sh with git_* names.
• Consolidate default-branch detection behind one implementation.
• Make producer-command failures distinguishable from an empty/not-found result; current process-substitution loops can hide git worktree list failure.
• Coordinate Base call-site/import updates before removing old names.
• Keep lib_gh.sh focused on GitHub remote parsing and gh CLI/API behavior.

Acceptance Criteria

• Git-only helpers live and are documented in the Git library.
• Base uses the new Git API without importing lib_gh.sh solely for local Git operations.
• Worktree/remote command failures return a real failure instead of empty success.
• No duplicate default-branch algorithm remains.

Validation

• Focused git/gh BATS coverage
• Relevant Base Bash tests for coordinated adoption
• ./tests/validate.sh
• git diff --check