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/agent/config.rs

@@ -2171,6 +2171,19 @@ impl Config {
         )
         .with_segment_detail(self.resolve_compaction_detail())
     }
+    /// Resolve summarizer `tool_choice`: env `KIGI_COMPACTION_TOOL_CHOICE` >
+    /// config > remote settings > default (`auto`).
+    pub(crate) fn resolve_compaction_tool_choice(
+        &self,
+    ) -> crate::util::config::CompactionToolChoice {
+        crate::util::config::resolve_compaction_tool_choice_from(
+            env_string(crate::util::config::ENV_COMPACTION_TOOL_CHOICE).as_deref(),
+            self.features.compaction_tool_choice.as_deref(),
+            self.remote_settings
+                .as_ref()
+                .and_then(|r| r.compaction_tool_choice.as_deref()),
+        )
+    }
     /// Resolve verbatim-input flag: env `KIGI_COMPACTION_VERBATIM_INPUT` > config > remote settings > default `true`.
     pub(crate) fn resolve_compaction_verbatim_input(&self) -> bool {
         BoolFlag::env("KIGI_COMPACTION_VERBATIM_INPUT")
@@ -3696,6 +3709,9 @@ pub struct Features {
     /// Feed the summarizer the verbatim conversation instead of the lossy rewrite; `None` = defer to env/remote settings/default (true).
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub compaction_verbatim_input: Option<bool>,
+    /// Summarizer `tool_choice` (`auto` | `none`); `None` = defer to env/remote settings/default (`auto`).
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub compaction_tool_choice: Option<String>,
     /// Snapshot a completed subagent's isolated worktree into a durable git ref
     /// and delete its directory (resume rehydrates from the ref). This is the
     /// per-deployment rollout lever (set in managed_config.toml `[features]`).