test(e2e): add dump-only-sql mixed-run + CLI contract coverage

- rename dump-only flag to --dump-only-sql across docs and tests
- update backup logic: skip files/ only for DB volumes when dumps succeed; fallback to files when dumps fail
- extend e2e helpers to support dump_only_sql
- add e2e mixed-run regression test (DB dump => no files/, non-DB => files/)
- add e2e CLI/argparse contract test (--dump-only-sql present, --dump-only rejected)
- fix e2e files test to expect file backups for non-DB volumes in dump-only-sql mode and verify restore
- update changelog + README flag table

https://chatgpt.com/share/69522d9c-ce08-800f-9070-71df3bd779ae

tests/e2e/test_e2e_cli_contract_dump_only_sql.py

@@ -0,0 +1,38 @@
+import unittest
+
+from .helpers import run
+
+
+class TestE2ECLIContractDumpOnlySql(unittest.TestCase):
+    def test_help_mentions_new_flag(self) -> None:
+        cp = run(["baudolo", "--help"], capture=True, check=True)
+        out = (cp.stdout or "") + "\n" + (cp.stderr or "")
+        self.assertIn(
+            "--dump-only-sql",
+            out,
+            f"Expected '--dump-only-sql' to appear in --help output. Output:\n{out}",
+        )
+
+    def test_help_does_not_mention_old_flag(self) -> None:
+        cp = run(["baudolo", "--help"], capture=True, check=True)
+        out = (cp.stdout or "") + "\n" + (cp.stderr or "")
+        self.assertNotIn(
+            "--dump-only",
+            out,
+            f"Did not expect legacy '--dump-only' to appear in --help output. Output:\n{out}",
+        )
+
+    def test_old_flag_is_rejected(self) -> None:
+        cp = run(["baudolo", "--dump-only"], capture=True, check=False)
+        self.assertEqual(
+            cp.returncode,
+            2,
+            f"Expected exitcode 2 for unknown args, got {cp.returncode}\n"
+            f"STDOUT={cp.stdout}\nSTDERR={cp.stderr}",
+        )
+        err = (cp.stderr or "") + "\n" + (cp.stdout or "")
+        # Argparse typically prints "unrecognized arguments"
+        self.assertTrue(
+            ("unrecognized arguments" in err) or ("usage:" in err.lower()),
+            f"Expected argparse-style error output. Output:\n{err}",
+        )