refactor: collapse GATEWAY_USER_ID into IRONCLAW_OWNER_ID

Remove the separate GATEWAY_USER_ID config. The gateway now uses
IRONCLAW_OWNER_ID (config.owner_id) directly for auth identity,
bootstrap user creation, and workspace scoping.

Previously, with_owner_scope() rebinds the auth identity to owner_id
while keeping default_sender_id as the gateway user_id. This caused
a FK constraint violation when creating users because the auth
identity ("default") didn't match any user in the DB ("nearai").

Changes:
- Remove GATEWAY_USER_ID env var and gateway_user_id from settings
- Remove user_id field from GatewayConfig
- Add owner_id parameter to GatewayChannel::new()
- Remove with_owner_scope() method
- Remove default_sender_id from GatewayState
- Remove sender override logic in chat/approval handlers
- Remove debug endpoint and tracing from prior debugging
- Update all tests and E2E fixtures

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

tests/support/gateway_workflow_harness.rs

@@ -227,7 +227,6 @@ impl GatewayWorkflowHarness {
             prompt_queue: None,
             scheduler: Some(scheduler_slot.clone()),
             owner_id: user_id.clone(),
-            default_sender_id: user_id.clone(),
             shutdown_tx: tokio::sync::RwLock::new(None),
             ws_tracker: Some(Arc::new(WsConnectionTracker::new())),
             llm_provider: Some(Arc::clone(&components.llm)),