Change engine/.env to be committed and have .env.local override (#6150)

# Description of Changes
We keep adding stuff to `engine/config/.env.example` and have to
manually update `.env` because of it, which is really clunky, especially
when working on multiple worktrees at once. This PR changes it so that
we just have a committed `.env` file and have an `.env.local` override
to put the actual private keys into, which should make it a bit easier
to manage.

> [!warning]
>
> After this goes in, be very careful for a little while not to
accidentally commit any keys that you've got inside your `.env` file!

engine/scripts/setup_env.py

@@ -1,48 +1,24 @@
 """
-Copies .env from .env.example if missing, and errors if any keys from the example
-are absent from the actual .env file.
+Ensures `.env.local` exists so developers have a place to put overrides
+(API keys, local model choices, etc.) without touching the committed `.env`.
 
 Usage:
     uv run scripts/setup_env.py
 """
 
-import os
-import shutil
-import sys
 from pathlib import Path
 
-from dotenv import dotenv_values
-
 ROOT = Path(__file__).parent.parent
-EXAMPLE_FILE = ROOT / "config" / ".env.example"
-ENV_FILE = ROOT / ".env"
+ENV_LOCAL_FILE = ROOT / ".env.local"
 
-print("setup-env: see engine/config/.env.example for documentation")
+TEMPLATE = """\
+###############################################################################
+# Local overrides for `engine/.env`
+# Put API keys and machine-specific settings here. Any variable defined here
+# takes precedence over the committed `.env`
+###############################################################################
+"""
 
-if not EXAMPLE_FILE.exists():
-    print(f"setup-env: {EXAMPLE_FILE.name} not found, skipping", file=sys.stderr)
-    sys.exit(0)
-
-if not ENV_FILE.exists():
-    shutil.copy(EXAMPLE_FILE, ENV_FILE)
-    print("setup-env: created .env from .env.example")
-
-env_keys = set(dotenv_values(ENV_FILE).keys()) | set(os.environ.keys())
-example_keys = set(dotenv_values(EXAMPLE_FILE).keys())
-missing = sorted(example_keys - env_keys)
-
-if missing:
-    sys.exit(
-        "setup-env: .env is missing keys from .env.example:\n"
-        + "\n".join(f"  {k}" for k in missing)
-        + "\n  Add them manually or delete your local .env to re-copy from config/.env.example."
-    )
-
-extra = sorted(k for k in dotenv_values(ENV_FILE) if k.startswith("STIRLING_") and k not in example_keys)
-if extra:
-    print(
-        "setup-env: .env contains STIRLING_ keys not in config/.env.example:\n"
-        + "\n".join(f"  {k}" for k in extra)
-        + "\n  Add them to config/.env.example if they are intentional.",
-        file=sys.stderr,
-    )
+if not ENV_LOCAL_FILE.exists():
+    ENV_LOCAL_FILE.write_text(TEMPLATE)
+    print("setup-env: created empty .env.local for local overrides")