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/acp_session_impl/spawn.rs

@@ -112,6 +112,7 @@ pub(crate) async fn spawn_session_actor(
     system_prompt_label: String,
     compaction_mode: kigi_chat_state::CompactionMode,
     compaction_verbatim_input: bool,
+    compaction_tool_choice: crate::util::config::CompactionToolChoice,
     two_pass_enabled: bool,
     buffering_settings: Option<BufferingSettings>,
     origin_client: Option<crate::http::OriginClientInfo>,
@@ -1032,6 +1033,7 @@ pub(crate) async fn spawn_session_actor(
             previous_model: std::cell::Cell::new(None),
             compaction_mode,
             verbatim_input: compaction_verbatim_input,
+            tool_choice: compaction_tool_choice,
             prefire: crate::session::compaction_config::PrefireState::default(),
             prefix_released: std::sync::atomic::AtomicBool::new(false),
         },
@@ -1535,6 +1537,7 @@ pub(crate) async fn spawn_session_on_thread(
     system_prompt_label: String,
     compaction_mode: kigi_chat_state::CompactionMode,
     compaction_verbatim_input: bool,
+    compaction_tool_choice: crate::util::config::CompactionToolChoice,
     two_pass_enabled: bool,
     buffering_settings: Option<BufferingSettings>,
     origin_client: Option<crate::http::OriginClientInfo>,
@@ -1679,6 +1682,7 @@ pub(crate) async fn spawn_session_on_thread(
                     system_prompt_label,
                     compaction_mode,
                     compaction_verbatim_input,
+                    compaction_tool_choice,
                     two_pass_enabled,
                     buffering_settings,
                     origin_client,