Tighten helper contracts

[codeforester]

Summary

• Make safe_mkdir option parsing and empty-argument behavior explicit.
• Allow BASE_GIT_PULL_MAX_ATTEMPTS to configure pull retry attempts.
• Refactor git_get_current_branch to use git -C and return empty-success for missing/non-Git directories.
• Document the tightened stdlib and git helper contracts.

Validation

• RED: focused safe_mkdir and git helper contract tests failed before the implementation.
• GREEN: bats --filter "safe_mkdir (warns|rejects)" lib/bash/std/tests/lib_std.bats
• GREEN: bats --filter "(missing directories|does not use pushd|configured max attempts)" lib/bash/git/tests/lib_git.bats
• bats lib/bash/std/tests/lib_std.bats
• bats lib/bash/git/tests/lib_git.bats
• shellcheck --severity=error lib/bash/std/lib_std.sh lib/bash/std/tests/lib_std.bats lib/bash/git/lib_git.sh lib/bash/git/tests/lib_git.bats
• git diff --check
• ./tests/validate.sh

Fixes #19