fix(doctor): prefer config dir for loaded plugin version

[code-yeongyu]

Summary

• check the OpenCode config install before the legacy cache install when resolving the loaded plugin version
• fall back to the cache install path when the config directory does not contain the plugin package
• add regression tests for config-dir precedence and legacy cache fallback

Testing

• bun test src/cli/doctor/
• bun run typecheck
• bun run build

---

Summary by cubic

Fixes doctor version detection to prefer the OpenCode config directory and fall back to the legacy cache, so bun-based installs report the correct loaded plugin version.

• Bug Fixes
  • Prefer OPENCODE_CONFIG_DIR; fall back to XDG_CACHE_HOME.
  • Tests: add config-dir precedence and legacy fallback; isolate module import to prevent CI mock leaks.

^{Written for commit 26ae247. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: The PR correctly implements precedence logic and fallback for plugin version detection in the doctor command, backed by thorough regression tests and no risk to core functionality.}