Tighten small helper contracts in stdlib and git libraries

[codeforester]

Context

Several smaller helper behaviors are not urgent defects, but they make the public API less tidy than it could be:

• safe_mkdir only handles a leading -p by hand instead of using option parsing consistent with the rest of stdlib.
• _git_pull_with_retry hardcodes two attempts.
• git_get_current_branch changes process directory with pushd/popd even though git -C can avoid that.
• git_get_current_branch docs say non-repo/missing directories store an empty string, but missing directories currently return nonzero.

Scope

• Make safe_mkdir option/no-arg handling explicit and tested.
• Add a documented way to configure git pull retry attempts, or document the fixed retry policy as intentional.
• Refactor git_get_current_branch to avoid changing the caller's working directory where practical.
• Align git_get_current_branch docs, return codes, and tests.

Acceptance Criteria

• Helper contracts are documented and covered by BATS tests.
• git_get_current_branch does not unexpectedly perturb the caller's directory stack.
• Retry behavior is either configurable or explicitly documented.
• Existing callers continue to work or have documented migration notes.

Validation

• ./tests/validate.sh

Suggested Branch

enhancement/<issue>-20260618-helper-contract-cleanup