feat: add capture-history command

[KumamuKuma]

Linked issue

Fixes #27.

Issue #27 asks for a bulk workflow to bootstrap research-git from existing repository history instead of repeatedly capturing old commits by hand or capturing one large blurry range.

Summary

• add rgit capture-history A..B for importing existing commit history one commit at a time
• default to dry-run planning, with --write to create open proposals
• skip empty/merge/oversized diffs, reuse duplicate open proposals, and expose JSON output for review
• document the workflow in README

Validation

• python -m pytest tests/test_capture_history.py -q -> 22 passed
• python -m pytest tests/test_capture_history.py tests/test_cli.py -k "capture_history or capture_range or capture_same_diff or pending_json or capture_auto or capture_trigger_commit" -q -> 30 passed, 100 deselected
• python -m pytest tests/test_gitutil.py -k "range_source or diff_of_commit or resolve_commit" -q -> 12 passed, 35 deselected
• python -m pytest tests/test_segmenter.py -k "not replaced_external_symlink" -q -> 16 passed, 1 deselected
• python -m pytest -q -k "not test_run_with_init_flag_bootstraps_store and not test_diff_since_captures_replaced_external_symlink_without_leaking and not test_cli_run_with_resolves_name_to_active_edge and not test_segment_diff_captures_replaced_external_symlink_without_leaking and not test_python_dash_m_rgit_entrypoint" -> 492 passed, 5 skipped, 5 deselected
• git diff --check

Native Windows full-suite failures observed before this branch are handled separately in #31.

[Lum1104]

Thanks, we are building this already