feat(db_store): decouple migration from store construction

Remove auto-migration from DbStore.__init__; add explicit check_migrations()
and migrate() methods. The CLI now checks both stores on every startup and
prompts the user to confirm before migrating. supervise.prepare() calls
.migrate() directly now that __init__ no longer does it implicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_cli_dispatch.py

@@ -12,11 +12,17 @@ from unittest.mock import patch
 
 import bot_bottle.cli as climod
 from bot_bottle.cli import main
+from bot_bottle.db_store import DbStore
 from bot_bottle.log import Die
 from bot_bottle.manifest import ManifestError
 
 
 class TestMainDispatch(unittest.TestCase):
+    def setUp(self) -> None:
+        patcher = patch.object(DbStore, "check_migrations", return_value=True)
+        self._mock_check = patcher.start()
+        self.addCleanup(patcher.stop)
+
     def test_no_args_prints_usage_returns_2(self) -> None:
         with patch("sys.stderr", io.StringIO()):
             self.assertEqual(2, main([]))