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/e2e/conftest.py

@@ -271,7 +271,6 @@ async def ironclaw_server(
         "GATEWAY_HOST": "127.0.0.1",
         "GATEWAY_PORT": str(gateway_port),
         "GATEWAY_AUTH_TOKEN": AUTH_TOKEN,
-        "GATEWAY_USER_ID": "e2e-web-sender",
         "HTTP_HOST": "127.0.0.1",
         "HTTP_PORT": str(http_port),
         "HTTP_WEBHOOK_SECRET": HTTP_WEBHOOK_SECRET,
@@ -371,7 +370,6 @@ async def hosted_oauth_refresh_server(
             "GATEWAY_HOST": "127.0.0.1",
             "GATEWAY_PORT": str(gateway_port),
             "GATEWAY_AUTH_TOKEN": AUTH_TOKEN,
-            "GATEWAY_USER_ID": OWNER_SCOPE_ID,
             "HTTP_HOST": "127.0.0.1",
             "HTTP_PORT": str(http_port),
             "HTTP_WEBHOOK_SECRET": HTTP_WEBHOOK_SECRET,
@@ -411,7 +409,6 @@ async def hosted_oauth_refresh_server(
             yield {
                 "base_url": base_url,
                 "db_path": db_path,
-                "gateway_user_id": OWNER_SCOPE_ID,
                 "mock_llm_url": mock_llm_server,
             }
         except TimeoutError:
@@ -473,7 +470,6 @@ async def http_channel_server_without_secret(
         "GATEWAY_HOST": "127.0.0.1",
         "GATEWAY_PORT": str(gateway_port),
         "GATEWAY_AUTH_TOKEN": AUTH_TOKEN,
-        "GATEWAY_USER_ID": "e2e-tester",
         "HTTP_HOST": "127.0.0.1",
         "HTTP_PORT": str(http_port),
         "CLI_ENABLED": "false",