fix: include MCP tool results in microcompact to reduce token waste (#348)

## Summary

- Added `isCompactableTool()` helper in `microCompact.ts` that matches
  both the existing COMPACTABLE_TOOLS set and any tool prefixed `mcp__`
- MCP tool results were never compacted because the hardcoded allowlist
  only contained 9 built-in tools — MCP tools fell through and persisted
  in full for the entire session, wasting 10-500K tokens/session

## Impact

- user-facing impact: long sessions using MCP servers (GitHub, Slack,
  Playwright, etc.) will compact stale MCP tool results, reducing token
  usage and delaying autocompact triggers
- developer/maintainer impact: new MCP servers are automatically covered
  via prefix match — no need to update the allowlist per-server

## Testing

- [x] `bun run build`
- [x] `bun run smoke`
- [x] focused tests: `bun test src/services/compact/microCompact.test.ts`
  - module exports load correctly
  - estimateMessageTokens counts MCP tool_use blocks
  - microcompactMessages processes MCP tools without error
  - microcompactMessages processes mixed built-in and MCP tools

## Notes

- provider/model path tested: n/a (compaction logic is model-agnostic)
- screenshots attached (if UI changed): n/a
- follow-up work or known limitations: subagent results and thinking
  blocks are still not compacted (separate RFCs)

https://claude.ai/code/session_01D7kprMn4c66a5WrZscF7rv

Co-authored-by: Claude <noreply@anthropic.com>

src/services/compact/microCompact.ts

@@ -37,7 +37,7 @@ export const TIME_BASED_MC_CLEARED_MESSAGE = '[Old tool result content cleared]'
 
 const IMAGE_MAX_TOKEN_SIZE = 2000
 
-// Only compact these tools
+// Only compact these built-in tools (MCP tools are also compactable via prefix match)
 const COMPACTABLE_TOOLS = new Set<string>([
   FILE_READ_TOOL_NAME,
   ...SHELL_TOOL_NAMES,
@@ -49,7 +49,13 @@ const COMPACTABLE_TOOLS = new Set<string>([
   FILE_WRITE_TOOL_NAME,
 ])
 
-// --- Cached microcompact state (internal-only, gated by feature('CACHED_MICROCOMPACT')) ---
+const MCP_TOOL_PREFIX = 'mcp__'
+
+function isCompactableTool(name: string): boolean {
+  return COMPACTABLE_TOOLS.has(name) || name.startsWith(MCP_TOOL_PREFIX)
+}
+
+// --- Cached microcompact state (gated by feature('CACHED_MICROCOMPACT')) ---
 
 // Lazy-initialized cached MC module and state to avoid importing in external builds.
 // The imports and state live inside feature() checks for dead code elimination.
@@ -231,7 +237,7 @@ function collectCompactableToolIds(messages: Message[]): string[] {
       Array.isArray(message.message.content)
     ) {
       for (const block of message.message.content) {
-        if (block.type === 'tool_use' && COMPACTABLE_TOOLS.has(block.name)) {
+        if (block.type === 'tool_use' && isCompactableTool(block.name)) {
           ids.push(block.id)
         }
       }