Fix check_patch_targets' own fail-open: put the repo root on sys.path

The pre-commit CI job reported all 19 patch targets stale, including ones no
commit in this sweep touched -- the tell that the checker, not the targets,
was broken.

Running it as `python3 scripts/check_patch_targets.py` puts *scripts/* on
sys.path[0], not the repo root. `diet_guard` is not installed in the runner's
environment (CI installs requirements.txt only), so every `import_module` hit
the resolver's `except ImportError: continue` and every target reported
unresolvable. It passed locally because the dev venv has the package on the
path.

That is precisely the fail-open shape this script exists to prevent, in the
script itself: a green-or-red signal that reflects the environment rather than
the code. Two fixes:

- put the repo root on sys.path explicitly, so the checker resolves the same
  way whatever cwd or interpreter invokes it;
- import the package once up front and fail with a distinct message if that
  fails, so "the package will not import" can never again masquerade as
  "every patch target moved".

Reproduced in a clean venv built from requirements.txt: 19 stale before, exit 0
after, and still exactly 1 stale when a target is genuinely broken
(verified by renaming FOOD_LOG_FILE's target).

Verified: 708 Python tests / 100% branch coverage, pre-commit clean, checker
exit 0 under both the dev venv and the clean venv.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RA9ynjqXPnexRQnR5oAuCC

app/pubspec.yaml

@@ -2,7 +2,7 @@ name: diet_guard_app
 description: "Companion phone app for diet_guard: log meals on the go."
 publish_to: 'none'
 
-version: 1.0.28+1
+version: 1.0.29+1
 
 environment:
   sdk: ^3.12.2

scripts/check_patch_targets.py

@@ -31,6 +31,14 @@ PACKAGE = "diet_guard"
 
 TESTS_ROOT = pathlib.Path(PACKAGE) / "tests"
 
+# Running this as `python3 scripts/check_patch_targets.py` puts *scripts/* on
+# sys.path, not the repo root -- so `import diet_guard` fails unless the
+# package happens to be installed. It is not, on a CI runner that only
+# installs requirements.txt. Without this the resolver's `except ImportError`
+# swallowed every lookup and the hook reported all 19 targets as stale, which
+# is the exact fail-open shape this script exists to prevent.
+sys.path.insert(0, str(pathlib.Path(__file__).resolve().parent.parent))
+
 
 def patch_targets(root: pathlib.Path) -> Iterator[tuple[pathlib.Path, int, str]]:
     """Yield ``(file, lineno, dotted_target)`` for every literal patch target."""
@@ -78,6 +86,20 @@ def main() -> int:
         sys.stderr.write(f"Error: {TESTS_ROOT} not found; run from the repo root.\n")
         return 1
 
+    # Distinguish "this target moved" from "the package will not import at
+    # all". Without this the second case masquerades as the first: every
+    # target reports stale at once, which reads like a catastrophic refactor
+    # and hides the real error (a missing dependency, a syntax error).
+    try:
+        importlib.import_module(PACKAGE)
+    except ImportError as exc:
+        sys.stderr.write(
+            f"Error: cannot import {PACKAGE} ({exc}); every patch target would "
+            f"be reported stale. Fix the import first -- this is an environment "
+            f"problem, not a stale patch target.\n",
+        )
+        return 1
+
     stale = [
         (path, lineno, target)
         for path, lineno, target in patch_targets(TESTS_ROOT)