fix: reject /add of files in a sibling dir sharing the root's name prefix

[nolanchic]

Problem

/add <absolute_path> could add a file that lives outside the repo root into the chat (and therefore into the LLM context) whenever the file lived in a sibling directory whose name started with the repo root's name.

Example: with the repo rooted at /tmp/x/repo, the following was accepted:

/add /tmp/x/repoXYZ/secret.env

because /tmp/x/repoXYZ/secret.env.startswith(/tmp/x/repo) is True (string prefix, no path separator).

This is the same class of out-of-tree leak that #178 addressed, but on the literal absolute-path branch of cmd_add — the glob branch (glob_filtered_to_repo) was already protected by Path.is_relative_to(), so the gap only showed up for explicit absolute paths / autocomplete-provided paths.

Root cause

aider/commands.py, inside cmd_add's containment guard:

if (
    not abs_file_path.startswith(self.coder.root)   # str prefix, no separator
    and not is_image_file(matched_file)
    and self.coder.auto_commits
):

self.coder.root has no trailing path separator, so startswith matches any sibling directory whose name shares the root's prefix.

Fix

Use a real path-containment check, matching what glob_filtered_to_repo already does:

if (
    not Path(abs_file_path).is_relative_to(self.coder.root)
    and not is_image_file(matched_file)
    and self.coder.auto_commits
):

Path.is_relative_to is available on all supported Python versions (3.10+).

Test plan

• Added test_cmd_add_abs_path_in_sibling_dir_with_prefix — reproduces the leak on the old code (fails on main) and passes after the fix.
• pytest tests/basic/test_commands.py — full file green (70 tests), including the existing out-of-root / absolute-path / read-only-external tests (test_cmd_add_from_outside_root, test_cmd_add_abs_filename, test_cmd_add_read_only_with_external_file, …).
• pre-commit run --files aider/commands.py tests/basic/test_commands.py — isort / black / flake8 / codespell all pass.

Relates to #178.

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