fix: resolve startup toast version detection for local and cached installs

[idoall]

Summary

• resolve startup toast version detection when OpenCode stores plugin metadata under versioned cache directories
• fall back to config-dir node_modules installs when cache metadata is not present in the legacy location
• recognize project-root opencode.json during auto-update checker config discovery so local file:// plugin installs are detected correctly

Testing

• bun test src/hooks/auto-update-checker/checker/cached-version.test.ts src/hooks/auto-update-checker/checker/plugin-entry.test.ts src/hooks/auto-update-checker/constants.test.ts

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Summary by cubic

Fixes startup toast version detection so the correct plugin version is shown for local installs, versioned caches, and user-config installs.

• Bug Fixes
  • Enumerates CACHE_DIR for versioned cache dirs (e.g., oh-my-openagent@latest) and reads package.json from node_modules or the package root.
  • Falls back to user config node_modules when cache entries are missing.
  • Prefers project-root opencode.json/opencode.jsonc over .opencode/opencode.json to detect local file:// plugins.
  • Supports both oh-my-opencode and oh-my-openagent during version resolution.

^{Written for commit da0a006. Summary will update on new commits.}

[github-actions]

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[idoall]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

No issues found across 5 files

Confidence score: 5/5

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

_{Auto-approved: The PR expands version detection to support versioned cache directories and project-root configs with robust error handling and tests, posing no regression risk.}