examples: simplify .env.example to match the TypeScript package

Byte-identical to examples/.env.example in the TypeScript package: the two API
keys you must set, plus optional LangSmith tracing. The model-override vars are
still read by the examples and documented in their READMEs, which is where
someone tuning one is already looking.

Author: shixish

examples/.env.example

@@ -1,14 +1,10 @@
+# Copy to .env and fill in. The examples need both keys; model overrides are
+# documented in each example's README.
 DIFFBOT_API_TOKEN=
 ANTHROPIC_API_KEY=
 
 # Optional: LangSmith tracing. Set TRACING=true and paste your key to enable the
-# "View trace in LangSmith" link in the dql_explorer example.
+# "View trace in LangSmith" link in the DQL Explorer example.
 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=