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>

steward/core/migrations.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+from pathlib import Path
+
+
+def get_core_head() -> str:
+    """Return the current head revision ID of the core migration scripts.
+
+    Called at migration authoring time (when running ``alembic revision``),
+    not at application runtime. The returned ID should be captured statically
+    in a plugin's ``depends_on`` field.
+    """
+    from alembic.config import Config
+    from alembic.script import ScriptDirectory
+
+    migrations_dir = Path(__file__).parent.parent / "migrations"
+    cfg = Config()
+    cfg.set_main_option("script_location", str(migrations_dir))
+    script = ScriptDirectory.from_config(cfg)
+    heads = script.get_heads()
+    if len(heads) != 1:
+        raise RuntimeError(
+            f"Expected exactly 1 core migration head, got {len(heads)}: {heads}"
+        )
+    return heads[0]