Reject stage2_model.stage1_context = 'sampled' as unimplemented (issues.md Issue 1)

trainers.py unconditionally trains stage 2 against the ground-truth
stage-1 output (stage1_ctx = x1_s1.detach()), but 'sampled' was accepted
by validate_config, stored in config.toml and the checkpoint's
model_config, and silently trained identically to 'truth' — mislabeling
every downstream artifact for a run launched with
--stage2-stage1-context sampled. Mirrors the existing stop_token
validate_config pattern. User chose the immediate fix (reject loudly)
over the proper fix (actually implement sampled context), which is
scoped to Issue 16.

Also updates the _KNOWN_UNUSED reason for stage2_model.stage1_context
(added by Issue 5's consumed-keys audit) to reflect that the value is
now rejected rather than silently accepted, and drops the now-invalid
--stage2-stage1-context sampled case from test_stage2_only_knobs (a
full CLI invocation) — that flag's plumbing is still covered at the
overrides-dict level by test_overrides_from_flags_stage2_only_knobs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_config.py

@@ -687,6 +687,15 @@ def test_validate_config_stop_token_not_implemented():
         assert "stop_token" in str(e)
 
 
+def test_validate_config_stage1_context_sampled_not_implemented():
+    cfg = _cfg_with(**{"stage2_model.stage1_context": "sampled"})
+    try:
+        gconfig.validate_config(cfg)
+        assert False, "expected ValueError"
+    except ValueError as e:
+        assert "sampled" in str(e)
+
+
 def test_validate_config_n_sec_truth_rejected_for_rollout_capable_checkpoint():
     """'n_sec.mode = "truth" is invalid for a rollout-capable checkpoint' —
     both stages active means giant rollout