feat(kvb-git-commit-next): stage a file by line with stage_hunks.py

stage_hunks.py takes git diff -U0 of one file, keeps the added lines of an add-only hunk that match a regex and every other hunk whole when one of its changed lines matches, and applies the result to the index with git apply --cached --unidiff-zero --recount, or prints it with --dry-run; exit 1 when nothing matches. Step 4 of kvb-git-commit-next names it for a file that carries two bundles, which replaces the hand-written partial patches needed five times on 2026-11-03. tests/test_stage_hunks.py covers matching, the unstaged rest and the exit codes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_stage_hunks.py

@@ -0,0 +1,65 @@
+"""Exercise stage_hunks.py: matching added lines, whole changed hunks, the rest stays unstaged."""
+
+from __future__ import annotations
+
+import shutil
+import subprocess
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+SCRIPT = REPO_ROOT / "skills" / "kvb-git-commit-next" / "scripts" / "stage_hunks.py"
+
+
+@unittest.skipUnless(shutil.which("git"), "git not installed")
+class TestStageHunks(unittest.TestCase):
+    def setUp(self):
+        self.tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(self.tmp.cleanup)
+        self.repo = Path(self.tmp.name)
+        self.env = {
+            "PATH": "/usr/bin:/bin",
+            "HOME": str(self.repo),
+            "GIT_AUTHOR_NAME": "t",
+            "GIT_AUTHOR_EMAIL": "t@t",
+            "GIT_COMMITTER_NAME": "t",
+            "GIT_COMMITTER_EMAIL": "t@t",
+        }
+        self.file = self.repo / "notes.txt"
+        self.file.write_text("one\ntwo\nthree\nfour\nfive\n", encoding="utf-8")
+        self._git("init", "-q")
+        self._git("add", "notes.txt")
+        self._git("commit", "-q", "-m", "base")
+        self.file.write_text(
+            "one\nalpha one\nbeta one\ntwo\nthree\nfour alpha\nfive\nbeta two\n", encoding="utf-8"
+        )
+
+    def _git(self, *args: str) -> str:
+        return subprocess.run(
+            ["git", *args], check=True, capture_output=True, text=True, env=self.env, cwd=self.repo
+        ).stdout
+
+    def _run(self, pattern: str) -> subprocess.CompletedProcess:
+        return subprocess.run(
+            [sys.executable, str(SCRIPT), "--repo", str(self.repo), "--file", "notes.txt", "--match", pattern],
+            check=False, capture_output=True, text=True, env=self.env,
+        )
+
+    def test_only_matching_changes_are_staged(self):
+        result = self._run("alpha")
+        self.assertEqual(result.returncode, 0, result.stderr)
+        staged = [line for line in self._git("diff", "--cached", "-U0").splitlines() if line[:1] in "+-" and line[:3] not in ("+++", "---")]
+        self.assertEqual(staged, ["+alpha one", "-four", "+four alpha"])
+        unstaged = [line for line in self._git("diff", "-U0").splitlines() if line.startswith("+") and not line.startswith("+++")]
+        self.assertEqual(unstaged, ["+beta one", "+beta two"])
+
+    def test_no_match_exits_1_and_bad_pattern_exits_2(self):
+        self.assertEqual(self._run("gamma").returncode, 1)
+        self.assertEqual(self._run("(").returncode, 2)
+        self.assertEqual(self._git("diff", "--cached"), "")
+
+
+if __name__ == "__main__":
+    unittest.main()