feat: global usernames and rename workspace to default_workspace

Users are global entities, not scoped to workspaces. This change:

Track A — Global usernames:
- Change iam_users_by_username to PRIMARY KEY (username), removing
  workspace from the lookup key
- Login looks up username globally, no workspace required
- Username uniqueness is enforced globally, not per-workspace
- Login -w now overrides the JWT workspace (session workspace)
  rather than selecting which user registry to search

Track B — Rename workspace to default_workspace:
- UserRecord.workspace → UserRecord.default_workspace
- Identity.workspace → Identity.default_workspace
- JWT claim "workspace" → "default_workspace"
- IamResponse.resolved_workspace → resolved_default_workspace
- WebSocket auth-ok frame field → default_workspace
- Socket clients read default_workspace from auth-ok
- _user_record_to_dict wire key → default_workspace
- CLI help text and output updated throughout
- Test files updated for renamed fields

tests/unit/test_gateway/test_socket_graceful_shutdown.py

@@ -28,7 +28,7 @@ TEST_CAP = "graph:write"
 def _valid_identity():
     return Identity(
         handle="test-user",
-        workspace="default",
+        default_workspace="default",
         principal_id="test-user",
         source="api-key",
     )