examples: document the model-override env vars in .env.example

COMPANY_RESEARCH_MODEL and DQL_EXPLORER_MODEL are both read by the example
agents but were never mentioned in the template, so the only way to discover
them was reading agent.py. Mirrors the same block in the TypeScript package.

Author: shixish

examples/.env.example

@@ -6,3 +6,9 @@ ANTHROPIC_API_KEY=
 LANGSMITH_TRACING=false
 LANGSMITH_API_KEY=
 LANGSMITH_PROJECT=dql-explorer
+
+# Optional: override the model each example uses. Both default to
+# anthropic:claude-haiku-4-5 — a fast, cheap authoring loop that stays clear of
+# the low input-tokens-per-minute caps on fresh Anthropic accounts.
+COMPANY_RESEARCH_MODEL=
+DQL_EXPLORER_MODEL=