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/subagent/mod.rs

@@ -397,6 +397,21 @@ impl SubagentSpawnContext {
             .resolve()
             .value
     }
+    /// Subagent summarizer `tool_choice`, mirroring
+    /// `Config::resolve_compaction_tool_choice` (env > config > remote
+    /// settings > default `auto`).
+    pub fn resolve_compaction_tool_choice(&self) -> crate::util::config::CompactionToolChoice {
+        crate::util::config::resolve_compaction_tool_choice_from(
+            crate::agent::config::env_string(crate::util::config::ENV_COMPACTION_TOOL_CHOICE)
+                .as_deref(),
+            self.agent_config
+                .as_ref()
+                .and_then(|c| c.features.compaction_tool_choice.as_deref()),
+            self.remote_settings
+                .as_ref()
+                .and_then(|r| r.compaction_tool_choice.as_deref()),
+        )
+    }
     /// Whether a completed subagent's worktree is snapshotted into a durable ref
     /// and its directory deleted. Resolution mirrors the other subagent gates
     /// (env > config > remote settings > default). Default `false` so it ships dark;