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/services/compact/microCompact.ts

@@ -38,7 +38,7 @@ export const TIME_BASED_MC_CLEARED_MESSAGE = '[Old tool result content cleared]'
 const IMAGE_MAX_TOKEN_SIZE = 2000
 
 // Only compact these built-in tools (MCP tools are also compactable via prefix match)
-const COMPACTABLE_TOOLS = new Set<string>([
+export const COMPACTABLE_TOOLS = new Set<string>([
   FILE_READ_TOOL_NAME,
   ...SHELL_TOOL_NAMES,
   GREP_TOOL_NAME,
@@ -51,7 +51,7 @@ const COMPACTABLE_TOOLS = new Set<string>([
 
 const MCP_TOOL_PREFIX = 'mcp__'
 
-function isCompactableTool(name: string): boolean {
+export function isCompactableTool(name: string): boolean {
   return COMPACTABLE_TOOLS.has(name) || name.startsWith(MCP_TOOL_PREFIX)
 }