fix(compaction): configurable summarizer tool_choice; wider overflow detection

Port of upstream grok-build 0.2.105 + 0.2.118.

- CompactionToolChoice (auto default | none), resolved env
  KIGI_COMPACTION_TOOL_CHOICE > [features].compaction_tool_choice >
  remote settings. Some OpenAI-compatible servers reject
  tool_choice:"none" with tools attached, failing every long-session
  compaction; auto keeps them working (the summarization prompt already
  forbids tool use). Threaded from agent + subagent spawn through
  CompactionConfig into generate_session_compact; the Responses gate
  counts hosted tools too, so a hosted-only request still carries the
  configured choice.
- is_context_length_error recognizes the "current message ... exceeds
  budget" phrasing, so that overflow is Fatal instead of burning the
  retry budget on a deterministic failure.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSy9S31pZNWXtRZNVJ1sDf

crates/codegen/kigi-shell/src/session/compaction.rs

@@ -212,6 +212,7 @@ impl SessionActor {
             &sampling_config,
             self.inference_idle_timeout,
             wall_clock_budget_secs,
+            self.compaction.tool_choice,
         )
         .await
         {
@@ -983,6 +984,7 @@ impl SessionActor {
             sampling_config.clone(),
             self.inference_idle_timeout,
             wall_clock_budget_secs,
+            self.compaction.tool_choice,
         );
         let observer =
             crate::session::helpers::full_replace_compaction::ShellFullReplaceObserver::new(
@@ -2198,6 +2200,7 @@ mod inline_auto_compact_flow_tests {
                 previous_model: std::cell::Cell::new(None),
                 compaction_mode: kigi_chat_state::CompactionMode::Transcript,
                 verbatim_input: true,
+                tool_choice: Default::default(),
                 prefire: crate::session::compaction_config::PrefireState::default(),
                 prefix_released: std::sync::atomic::AtomicBool::new(false),
             },