Every reader's text a crew brief takes in has a test that it arrives as untrusted data

Part of #10441, the last piece of #10451, following #10365.

Done:

[ ] test/brief-injection.test.ts covers the brief for each of these, with the hostile body: the assigned issue from a reader, reader comments on an issue, feedback comments and reviews, a reader's line comment, a reader's conversation on a pull, and a foreign pull's diff. Each test fails if the text lands outside its fence or without its untrusted label.
[ ] docs/ops/RED-TEAM.md gets a section for crew briefs: each way text gets in, what holds it, and the test.

Files: test/brief-injection.test.ts, docs/ops/RED-TEAM.md

Every reader's text a crew brief takes in has a test that it arrives as untrusted data #10556