T02: Add GitIO kernel and diff fixtures

[NagisaVon]

Summary

• add the typed Git runner and error mapping boundary
• add object-only plumbing helpers for object, tree, commit, ref, and remote operations
• parse zero-context no-rename diffs with text and whole-file change flags
• add reusable real-Git scratch repositories, golden diff cases, and canonical isolation checks

Validation

• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv run pytest
• isolated pytest runs on Python 3.11 and 3.13

Scope

Implements T02 only. No T03+ models, storage, projection, publishing, CLI verbs, GitHub backend, or release workflow.