Summary

• treat any all-zero object ID as the null push sentinel in pre-push
• keep branch deletion short-circuits working for SHA-256 repos
• add regression coverage for 64-zero delete lines in both _run_ns() and hook_impl()

Closes #3664.

Validation

• baseline repro on current main: SHA-256 pre-push delete exited non-zero and raised CalledProcessError from git diff ...<64 zeros>
• .venv/bin/python -m pytest tests/commands/hook_impl_test.py -k 'sha256_zero_oid' -q
• .venv/bin/python -m pytest tests/commands/hook_impl_test.py -k 'pre_push' -q
• direct SHA-256 shell repro after fix: exit status 0

Gates

• E1 baseline repro exists: yes
• E2 new regression tests fail on baseline: yes
• E3 new regression tests pass after fix: yes
• E4 nearby targeted suite passes: yes
• E5 diff localized to pre_commit/commands/hook_impl.py and tests/commands/hook_impl_test.py: yes