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>

giant/config.py

@@ -1256,6 +1256,14 @@ def validate_config(cfg: dict) -> None:
             "(standalone stage-2 evaluation only, never for rollout)"
         )
 
+    if _get_path(cfg, "stage2_model.stage1_context") == "sampled":
+        raise ValueError(
+            "stage2_model.stage1_context = 'sampled' is accepted by the schema "
+            "but not implemented — trainers.py always trains stage 2 against "
+            "the ground-truth stage-1 output; use 'truth' (default) instead "
+            "(see issues.md Issue 16 for the planned implementation)"
+        )
+
     if (
         _get_path(cfg, "stage2_model.n_sec.mode") == "truth"
         and _get_path(cfg, "stage1_model.active")