fix(opencode): serialize config bun installs

[nexxeln]

Summary

• serialize config dependency installs with the existing bun-install lock so .opencode and temp config dirs do not run bun install --no-cache concurrently
• add a regression test that verifies concurrent config installs are executed one at a time
• fixes the intermittent windows e2e failure where Bun errored with ENOENT for zod during concurrent installs

Testing

• bun test test/config/config.test.ts --test-name-pattern 'serializes concurrent config dependency installs|installs dependencies in writable OPENCODE_CONFIG_DIR'
• bun typecheck