#18418: fix(plugins): improve duplicate plugin ID warning with actionable guidance

## Summary - Improve duplicate plugin ID warning messages with actionable precedence guidance - Show both conflicting source paths (not just the later one) - Include which copy takes precedence and a hint to remove the unintended one - Add test coverage for same-origin duplicate behavior ## Motivation As noted in maintainer feedback, the duplicate plugin warning is the **only** debugging signal when users unintentionally install a bundled plugin separately. This PR preserves the warning while making it significantly more actionable. ## Test plan - [x] Existing tests pass (symlink dedup, same-path dedup, precedence selection) - [x] New test: different-precedence duplicates include "takes precedence" guidance - [x] New test: same-origin duplicates warn without precedence hint - [x] `pnpm build` passes - [x] `pnpm test -- --run src/plugins/manifest-registry.test.ts` — 5/5 pass