test: add memory database tests and update conftest for DB isolation

conftest patches mdb._DB_PATH and calls init_db() after directory creation
so all existing tests continue to work with the new DB layer. New
test_memory_db.py covers upsert, search, remove, migration, and the
updated list_memories/lookup_memories integration paths.

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

tests/conftest.py

@@ -28,6 +28,7 @@ def tmp_pyra_home(tmp_path, monkeypatch):
 
     import pyra.plugins.loader as pl
     import pyra.plugins.executor as pe
+    import pyra.memory.database as mdb
 
     b.VAULT_PATH = fake_home / "vault"
     b.BLOCKED_PREFIXES = [b.VAULT_PATH]
@@ -35,6 +36,8 @@ def tmp_pyra_home(tmp_path, monkeypatch):
     mi._INDEX_FILE = fake_home / "memory" / "MEMORY_INDEX.md"
     mr._MEMORY_ROOT = fake_home / "memory"
     mw._MEMORY_ROOT = fake_home / "memory"
+    mdb._DB_PATH = fake_home / "memory" / "memory.db"
+    mdb._MEMORY_ROOT = fake_home / "memory"
     vr._KEYS_FILE = fake_home / "vault" / "secrets" / "api_keys.json"
     vw._KEYS_FILE = fake_home / "vault" / "secrets" / "api_keys.json"
     si._LOG_FILE = fake_home / "security.log"
@@ -52,6 +55,8 @@ def tmp_pyra_home(tmp_path, monkeypatch):
     (fake_home / "plugins").mkdir()
     (fake_home / "logs").mkdir()
 
+    mdb.init_db()
+
     # Reset plugin registry singleton so tests don't share state
     from pyra.plugins.registry import PluginRegistry
     PluginRegistry.reset()