tests(cli[sync_sigint]) Survive distro relative-PYTHONPATH builds (#549)

The single subprocess-based test in vcspull's CLI suite failed on
distro builders (e.g. Arch Linux PKGBUILD) that test staged-install
wheels through a relative ``PYTHONPATH`` entry from the source tree.
The root ``conftest.py``'s autouse ``cwd_default`` fixture chdirs each
test into a per-test ``tmp_path``, the subprocess inherits that CWD,
and the relative ``PYTHONPATH`` then resolves to a non-existent path
-- the child fails with ``ModuleNotFoundError: No module named
'vcspull'``. ``uv run pytest`` masks the bug because the venv exposes
vcspull via absolute site-packages.

- **Anchor** the subprocess to vcspull's actually-loaded install
  location via ``importlib.util.find_spec``, then prepend that
  absolute directory to the child's ``PYTHONPATH``.
- **Preserve** any incoming ``PYTHONPATH`` so other build setups
  remain unaffected.
- **Audit** the rest of the codebase for the same failure mode --
  this is the only test that spawns a Python subprocess importing
  vcspull, so the fix closes the full exposure surface.
- **Document** the packager-visible behaviour change under the
  CHANGES ``### Development`` section so distros can confirm the
  test-suite portability fix in release notes.

Author: tony

CHANGES

@@ -37,6 +37,15 @@ $ uvx --from 'vcspull' --prerelease allow vcspull
 _Notes on upcoming releases will be added here_
 <!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
 
+### Development
+
+#### Test suite works under distro relative-`PYTHONPATH` builds (#549)
+
+Distributions that test built wheels by staging them into a directory
+and pointing pytest at that directory via a relative `PYTHONPATH` entry
+from the source tree (the common Arch Linux PKGBUILD pattern) can now
+run vcspull's full test suite end to end.
+
 ## vcspull v1.60.0 (2026-05-18)
 
 vcspull v1.60.0 is a maintenance release for the May 2026 docs and

tests/cli/test_sync_sigint.py

@@ -2,7 +2,9 @@
 
 from __future__ import annotations
 
+import importlib.util
 import os
+import pathlib
 import signal
 import subprocess
 import sys
@@ -55,7 +57,24 @@ def test_exit_on_sigint_produces_wifsignaled_sigint() -> None:
         "    _exit_on_sigint()\n"
     )
 
-    env = {**os.environ, "PYTHONDONTWRITEBYTECODE": "1"}
+    # Pin the child's import path to wherever the parent loaded vcspull from.
+    # The root conftest's autouse ``cwd_default`` chdirs every test into a
+    # per-test ``tmp_path``, which the subprocess inherits as its CWD. Build
+    # environments that hand vcspull to pytest via a *relative* ``PYTHONPATH``
+    # entry (e.g. Arch's ``tmp_install/usr/lib/pythonX.Y/site-packages``)
+    # would then resolve that entry against the tmp dir and fail to import
+    # vcspull. Prepending the parent's resolved package dir keeps the child
+    # importable regardless of the surrounding install style.
+    vcspull_spec = importlib.util.find_spec("vcspull")
+    assert vcspull_spec is not None and vcspull_spec.origin is not None
+    vcspull_parent = str(pathlib.Path(vcspull_spec.origin).resolve().parent.parent)
+    env = {
+        **os.environ,
+        "PYTHONDONTWRITEBYTECODE": "1",
+        "PYTHONPATH": os.pathsep.join(
+            p for p in (vcspull_parent, os.environ.get("PYTHONPATH", "")) if p
+        ),
+    }
 
     proc = subprocess.run(
         [sys.executable, "-c", runner],