feat: check the profiles a branch adds or edits

profile_sync gains --changed (added or edited profiles, uncommitted
ones included) and --added, both against the merge base of --base
(origin/main by default). CONTRIBUTING runs --changed --strict instead
of one --emulator call per new profile, and the CI job runs --added
in place of its own diff loop.

tests/test_profile_sync.py

@@ -4261,8 +4261,9 @@ class TestSelectionNamesWhatMatchedNothing(unittest.TestCase):
     }
 
     def _args(self, **values):
-        base = {"emulator": None, "system": None, "type_": None,
-                "stale_before": None, "limit": None}
+        base = {"emulator": None, "changed": False, "added": False,
+                "system": None, "type_": None, "stale_before": None,
+                "limit": None}
         return argparse.Namespace(**{**base, **values})
 
     def test_a_misspelt_system_stops_and_suggests(self):
@@ -4610,5 +4611,72 @@ class TheTriageCountsWhatBlocksThePin(unittest.TestCase):
         self.assertEqual(len(err.getvalue().splitlines()), 1, err.getvalue())
 
 
+class TestBranchProfiles(unittest.TestCase):
+    """--changed and --added name the profiles a branch brings, not by hand."""
+
+    def setUp(self):
+        self._tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(self._tmp.cleanup)
+        self.root = Path(self._tmp.name)
+        self._cwd = os.getcwd()
+        self.addCleanup(os.chdir, self._cwd)
+        os.chdir(self.root)
+        env = {**os.environ, "GIT_AUTHOR_NAME": "t", "GIT_AUTHOR_EMAIL": "t@t",
+               "GIT_COMMITTER_NAME": "t", "GIT_COMMITTER_EMAIL": "t@t"}
+        self.env = env
+        self._git("init", "-q", "-b", "main")
+        (self.root / "emulators").mkdir()
+        (self.root / "emulators" / "kept.yml").write_text("a: 1\n")
+        (self.root / "emulators" / "edited.yml").write_text("a: 1\n")
+        self._git("add", "emulators")
+        self._git("commit", "-q", "--no-verify", "-m", "base")
+        self._git("checkout", "-q", "-b", "topic")
+        (self.root / "emulators" / "edited.yml").write_text("a: 2\n")
+        (self.root / "emulators" / "new.yml").write_text("a: 1\n")
+        self._git("add", "emulators")
+        self._git("commit", "-q", "--no-verify", "-m", "topic")
+        (self.root / "emulators" / "draft.yml").write_text("a: 1\n")
+        (self.root / "emulators" / "notes.txt").write_text("x\n")
+
+    def _git(self, *args):
+        subprocess.run(["git", *args], cwd=self.root, env=self.env, check=True)
+
+    def test_changed_takes_added_edited_and_uncommitted(self):
+        self.assertEqual(
+            profile_sync.branch_profiles("emulators", "main", added_only=False),
+            ["draft", "edited", "new"],
+        )
+
+    def test_added_leaves_edited_profiles_out(self):
+        self.assertEqual(
+            profile_sync.branch_profiles("emulators", "main", added_only=True),
+            ["draft", "new"],
+        )
+
+    def test_the_flag_selects_those_profiles(self):
+        args = profile_sync.build_parser().parse_args(["--changed", "--base", "main"])
+        profiles = {"kept": {}, "edited": {}, "new": {}, "draft": {}}
+        self.assertEqual(
+            list(profile_sync.select_profiles(profiles, args)),
+            ["draft", "edited", "new"],
+        )
+
+
+class TestBranchCheckIsWired(unittest.TestCase):
+    """CONTRIBUTING and CI name the branch's profiles through the flag."""
+
+    def test_contributing_and_ci_use_the_flags(self):
+        root = Path(__file__).resolve().parents[1]
+        sys.path.insert(0, str(root / "scripts"))
+        import generate_readme  # noqa: PLC0415
+
+        text = generate_readme.generate_contributing()
+        self.assertIn("profile_sync.py --changed --strict", text)
+        self.assertNotIn("--emulator NAME --strict", text)
+        workflow = (root / ".github" / "workflows" / "validate.yml").read_text()
+        self.assertIn("profile_sync.py --added --base", workflow)
+        self.assertNotIn('--emulator "$name"', workflow)
+
+
 if __name__ == "__main__":
     unittest.main()