chore: rename project Roundtable → Steward

Renames the Python package directory, CLI command, env var prefix,
docker-compose service/container/image, Postgres role/db, and all
visible branding. Marketing form is "Fabled Steward".

Clean break from the previous rebrand: drops the fabledscryer→roundtable
import shim in __init__.py and the FABLEDSCRYER_* env var fallback in
config.py and migrations/env.py. Env vars are now STEWARD_* only.

Heads-up for existing deployments:
- Postgres user/db renamed fabledscryer → steward in docker-compose.yml.
  Existing volumes need the role/db renamed inside Postgres, or override
  POSTGRES_USER/POSTGRES_DB to keep the old names.
- Host-agent systemd unit is now steward-agent.service. Existing agents
  keep running under the old name; reinstall to switch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/plugins/host_agent/test_agent_config.py

@@ -7,12 +7,12 @@ from plugins.host_agent.agent import read_config, ConfigError
 def test_parses_flat_key_value(tmp_path):
     p = tmp_path / "agent.conf"
     p.write_text(
-        "url = https://roundtable.example\n"
+        "url = https://steward.example\n"
         "token = abc123\n"
         "interval_seconds = 45\n"
     )
     cfg = read_config(str(p))
-    assert cfg["url"] == "https://roundtable.example"
+    assert cfg["url"] == "https://steward.example"
     assert cfg["token"] == "abc123"
     assert cfg["interval_seconds"] == 45