fix: make tests resilient to project .beads/redirect

Tests were failing because beads.FindDatabasePath() follows the
project's .beads/redirect file, causing tests to find unexpected
databases. Fixed by:

- Setting BEADS_DIR in tests that need isolation from git repo detection
- Clearing BEADS_DIR in TestMain to prevent global contamination
- Updating migration test schema to include owner column

This ensures tests work correctly in crew directories that have
redirect files pointing to shared .beads directories.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: beads/crew/dave
Rig: beads
Role: crew

cmd/bd/test_repo_beads_guard_test.go

@@ -29,6 +29,17 @@ func TestMain(m *testing.M) {
 		}
 	}()
 
+	// Clear BEADS_DIR to prevent tests from accidentally picking up the project's
+	// .beads directory via git repo detection when there's a redirect file.
+	// Each test that needs a .beads directory should set BEADS_DIR explicitly.
+	origBeadsDir := os.Getenv("BEADS_DIR")
+	os.Unsetenv("BEADS_DIR")
+	defer func() {
+		if origBeadsDir != "" {
+			os.Setenv("BEADS_DIR", origBeadsDir)
+		}
+	}()
+
 	if os.Getenv("BEADS_TEST_GUARD_DISABLE") != "" {
 		os.Exit(m.Run())
 	}