fix: create new files in the subtree cwd under --subtree-only

[rodboev]

Summary

Make subtree-only sessions create new relative files inside the current cwd subtree instead of the git root. This covers both unmatched /add paths and model-created files, while keeping tracked-file and non-subtree path semantics unchanged.

Why

--subtree-only already scopes visibility to the current subtree, but new-file creation still resolves from repo root. In monorepos that forces files into the wrong directory even when the whole session is intentionally scoped to a subproject.

Attribution

Scope follows gregid's monorepo report quoted in #3329 from #312 (comment).

Tests

• python -m pytest tests/basic/test_commands.py::TestCommands::test_cmd_add_subtree_only_creates_unmatched_file_in_cwd tests/basic/test_coder.py::TestCoder::test_allowed_to_edit_subtree_only_creates_new_file_in_cwd tests/basic/test_coder.py::TestCoder::test_allowed_to_edit_without_subtree_only_keeps_repo_root_for_new_files tests/basic/test_coder.py::TestCoder::test_allowed_to_edit_subtree_only_keeps_repo_relative_subtree_paths tests/basic/test_repo.py::TestRepo::test_subtree_only_new_file_path_resolution -v
• pre-commit run --all-files

Closes #3329

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