fix(disposal): Complete Instance disposal chain for plugins and caches

[sauerdaniel]

Summary

Complete the Instance disposal chain to ensure plugins and cached resources are properly cleaned up when instances are disposed.

Fixes #9157 Depends on #9141 (LRU cache utility)

Note: This PR builds on the LRU cache utility from #9141. Please merge that PR first.

Problem

When Instance.dispose() is called:

1. Plugins not disposed - Plugins may hold resources (connections, file handles) that need cleanup
2. Cached instances not disposed - The instance cache doesn't call disposal hooks when evicting entries
3. No disposal hook in Hooks interface - Plugins have no way to register cleanup logic

Solution

1. Plugin Dispose Hook

Add dispose() method to plugin Hooks interface.

2. LRU Cache onEvict Callback

Use the LRU cache's onEvict callback to dispose instances when evicted.

3. Bootstrap Cleanup

Add cleanup in bootstrap to dispose plugins before process exit.

Changes

• packages/plugin/src/index.ts - Add dispose() to Hooks interface
• packages/opencode/src/plugin/index.ts - Call plugin dispose() on instance disposal
• packages/opencode/src/project/instance.ts - Add onEvict callback for cache eviction
• packages/opencode/src/project/bootstrap.ts - Add cleanup for plugins on exit
• packages/opencode/src/provider/provider.ts - Use LRU cache for SDK instances
• packages/opencode/src/util/cache.ts - Add iterator support (from #9141)

Testing

• [x] TypeScript compilation passes (bun turbo typecheck)
• [x] Unit tests pass (725 tests, 0 failures)
• [x] Instance/project tests pass

Note: Manual plugin disposal testing was not performed. This requires creating a test plugin with cleanup logic and verifying it's called.

Stacking

This PR includes commits from #9141 (LRU cache utility). After #9141 is merged, this PR can be rebased to only include the disposal-specific changes.