pr-review-toolkit README Contributing section points to the private claude-cli-internal repo

The Contributing section of plugins/pr-review-toolkit/README.md (lines 297-301) tells readers the review agents are maintained in:

- User agents: `~/.claude/agents/`
- Project agents: `.claude/agents/` in claude-cli-internal

claude-cli-internal is not a public repository, and neither path exists for users of this repo. The six agents actually live in plugins/pr-review-toolkit/agents/ right here, so the section sends would-be contributors to a dead end. This looks like a leftover from before the plugin was open-sourced; it is the only reference to claude-cli-internal under plugins/.

Verified against main: ls plugins/pr-review-toolkit/agents/ lists all six agent files, and grep -rn 'claude-cli-internal' plugins/ matches only this README line. Happy to open a PR pointing the section at the in-repo directory.