feat(cli): clarify agent flags and support env defaults (#9063)

Agent deployment options now print a private-beta notice. Rename
`--environment` to `--agent-environment` and accept `LANGSMITH_AGENT_ID`
/ `LANGSMITH_AGENT_ENVIRONMENT` as process-environment defaults for
deploy and list. Explicit flags take precedence, and the backend payload
is unchanged.

Validation: formatting and lint pass. A local smoke check verified
environment-only deployment, explicit flag precedence, list defaults,
and structured JSON output. Full CLI suite: 411 passed; the two known
Docker failures remain (`test_dockerfile_command_with_docker_compose`
and `test_build_generate_proper_build_context`). No new tests added; the
existing test invocation uses the renamed flag.

libs/cli/tests/unit_tests/test_deploy_agents.py

@@ -58,7 +58,7 @@ AGENT_ARGS = [
     "deploy",
     "--agent-id",
     "customer-support",
-    "--environment",
+    "--agent-environment",
     "staging",
     "--remote",
     "--no-wait",