feat(util): Add bounded LRU cache utility

[sauerdaniel]

Summary

Add a bounded LRU (Least Recently Used) cache utility with eviction callbacks to prevent unbounded memory growth.

Fixes #9143

Problem

Several places in the codebase use unbounded Map objects for caching:

• Instance cache in project/instance.ts
• Provider SDK cache in provider/provider.ts

These can grow without limit in long-running processes or when handling many directories/providers.

Solution

Add a reusable createLruCache utility that:

• Limits cache size with maxEntries option
• Evicts least-recently-used entries when full
• Provides onEvict callback for cleanup logic
• Maintains Map-like interface for easy adoption

Changes

• packages/opencode/src/util/cache.ts - New LRU cache utility with:
  • maxEntries limit (default: Infinity for backward compatibility)
  • onEvict callback for disposal logic
  • LRU tracking via lastAccess timestamp
  • Iterator support for for...of loops

Testing

• [x] TypeScript compilation passes (bun turbo typecheck)
• [x] Unit tests pass (725 tests, 0 failures)
• [x] Cache utility has 36% line coverage from existing tests

Note: Manual memory testing (monitoring heap growth over time) was not performed.