feat(api): compress old tool_result content for small-context providers (#801)

* feat(api): compress old tool_result content for small-context providers

Adds a shim-layer pass that tiers tool_result content by age on
providers
  with small effective context windows (Copilot gpt-4o 128k, Mistral,
  Ollama). Recent turns remain full; mid-tier results are truncated to
2k
  chars; older results are replaced with a stub that preserves tool name
  and arguments so the model can re-invoke if needed.

  Tier sizes auto-tune via getEffectiveContextWindowSize, same
calculation
  used by auto-compact. Reuses COMPACTABLE_TOOLS and
  TOOL_RESULT_CLEARED_MESSAGE to complement (not duplicate)
microCompact.
  Configurable via /config toolHistoryCompressionEnabled.

  Addresses active-session context accumulation on Copilot where
  microCompact's time-based trigger never fires, which surfaces as
  "tools appearing in a loop" and prompt_too_long errors after ~15
turns.

* fix: config tool history

src/components/Settings/Config.tsx

@@ -281,6 +281,24 @@ export function Config({
         enabled: autoCompactEnabled
       });
     }
+  }, {
+    id: 'toolHistoryCompressionEnabled',
+    label: 'Tool history compression',
+    value: globalConfig.toolHistoryCompressionEnabled,
+    type: 'boolean' as const,
+    onChange(toolHistoryCompressionEnabled: boolean) {
+      saveGlobalConfig(current => ({
+        ...current,
+        toolHistoryCompressionEnabled
+      }));
+      setGlobalConfig({
+        ...getGlobalConfig(),
+        toolHistoryCompressionEnabled
+      });
+      logEvent('tengu_tool_history_compression_setting_changed', {
+        enabled: toolHistoryCompressionEnabled
+      });
+    }
   }, {
     id: 'spinnerTipsEnabled',
     label: 'Show tips',
@@ -1158,6 +1176,9 @@ export function Config({
     if (globalConfig.autoCompactEnabled !== initialConfig.current.autoCompactEnabled) {
       formattedChanges.push(`${globalConfig.autoCompactEnabled ? 'Enabled' : 'Disabled'} auto-compact`);
     }
+    if (globalConfig.toolHistoryCompressionEnabled !== initialConfig.current.toolHistoryCompressionEnabled) {
+      formattedChanges.push(`${globalConfig.toolHistoryCompressionEnabled ? 'Enabled' : 'Disabled'} tool history compression`);
+    }
     if (globalConfig.respectGitignore !== initialConfig.current.respectGitignore) {
       formattedChanges.push(`${globalConfig.respectGitignore ? 'Enabled' : 'Disabled'} respect .gitignore in file picker`);
     }