Fix incremental update leaving stale nodes after a file rename

[thejesh23]

Linked issue

Closes #684

What & why

get_changed_files() discovered changes with git diff --name-only, which reports a rename a.py → b.py as only the new path b.py. incremental_update() therefore never saw a.py, and its purge loop only removes paths that are in the changed/dependent set and missing on disk — the old path is in neither. Unlike full_build() (which purges existing_files - current_abs), the incremental path had no "purge files no longer on disk" step, so the renamed-away file's File/Function/Class nodes and every edge keyed to it survived. The incremental graph diverged from a full rebuild.

This switches change discovery to git diff --name-status -z and adds _parse_name_status_z(), which emits both the old and new path for rename/copy (R/C) records. The old path then flows through the existing purge loop and is removed, so an incremental update again equals a full rebuild. The --cached fallback branch gets the same treatment. The now-unused _decode_nul_paths() helper is removed.

How it was tested

uv run pytest tests/test_incremental.py tests/test_integration_git.py -q   # 85 passed
uv run pytest tests/ --tb=short -q   # 1962 passed (1 pre-existing Windows/py3.14 failure, unrelated)
uv run ruff check code_review_graph/
uv run mypy code_review_graph/incremental.py --ignore-missing-imports --no-strict-optional   # Success

New tests (both fail on main, pass here):

• test_incremental.py::TestGitOperations::test_get_changed_files_reports_both_rename_paths — unit test of the name-status parser.
• test_integration_git.py::test_incremental_update_purges_renamed_source — real-git test asserting the renamed-away source is purged and node/edge/file counts match a fresh full_build.

Checklist

• Tests added for new functionality
• All tests pass: uv run pytest tests/ --tb=short -q (one pre-existing, unrelated Windows/py3.14 failure)
• Linting passes: uv run ruff check code_review_graph/
• Type checking passes: uv run mypy code_review_graph/incremental.py --ignore-missing-imports --no-strict-optional
• Lines are at most 100 characters
• Docs updated where behavior changed (docstrings on the new/changed functions)