README: say reviewer findings need a plugin forge (GitHub Labs get no review comments) #385

Problem

The README's early-release note (README.md:32) says findings from outside reviewers "come back as fix rounds that you approve before anything is pushed". It doesn't say this works only on a plugin forge such as GitLab.

On a GitHub Lab, every collection fails with "this forge does not serve review comments". The Findings tab never fills, so a GitHub user who reads the README and adds a reviewer gets nothing. docs/install.md:129-130 and plugins/forgecomments/README.md both state the limit; the README is the only place that leaves it out.

Found while walking through the Findings tab on Molecule 359 (PR #374) in a scratch copy of the Lab.

Change

• Add one short clause to the README note saying outside reviewers need a plugin forge, and that GitHub Labs can't serve review comments yet. Link to the install.md section.
• lab/internal/ui/molecule_findings_ui_test.go:266 bans some phrases, such as "outside reviewers are not supported yet". Word the caveat so it says what works instead of calling the feature unsupported.

Done when

• The README says GitHub Labs don't get reviewer comments yet, in the same place it promises fix rounds.
• go test ./lab/internal/ui/... passes.