fix(spreadsheetbench)+optimizer: fix verify-feedback bloat, drop optimizer-side truncation, soft-disable gate

A. SpreadsheetBench verification-feedback bloat
   - rollout.py _auto_verify_output: use official _compare_cell_value (was
     repr() equality, which falsely flagged 5 vs 5.0 / None vs ""); collapse
     correct-and-empty cells into a count so large sparse answer ranges no
     longer flood feedback with MBs of None=None noise.
   - codegen_agent.py _build_eval_feedback: only list WRONG cells, collapse
     correct ones into a count.
   Scoring is unaffected (evaluate() is independent); this only fixes the
   target model's multi-turn solving feedback.

B. Remove optimizer-side truncation (bloat source now fixed)
   - reflect.py: drop _MAX_TRAJ_CHARS cap and all per-field clips.
   - update_modes.py / clip.py / lr_autonomous.py: describe_item /
     short_item_summary no longer truncate; raise ranking/lr token budget.
   - trainer.py _format_step_buffer: full task_ids / target.
   - slow_update.py: full comparison samples.

C. Soft-disable gate
   - config.py / trainer.py: use_gate=false no longer raises; validation still
     runs but candidates are force-accepted (new force_accept branch + log).

Misc: aggregate.py merge token budget 4096 -> 16384.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

skillopt/config.py

@@ -189,13 +189,6 @@ def flatten_config(cfg: dict) -> dict:
 
     flat: dict[str, Any] = {}
 
-    evaluation_section = cfg.get("evaluation", {})
-    if isinstance(evaluation_section, dict) and evaluation_section.get("use_gate") is False:
-        raise ValueError(
-            "Gate validation is mandatory in this branch. Remove "
-            "`evaluation.use_gate: false` from the config."
-        )
-
     # Apply the explicit mapping
     for dotted, flat_key in _FLATTEN_MAP.items():
         section, key = dotted.split(".", 1)