Crew briefs point at docs/PRINCIPLES.md instead of restating it #8737

Left open from #7641 (pull #8689, 82bd2cb6): the principles file exists and the briefs read it, but CLAUDE.md and the brief text still restate several of the same principles. The owner's ask was that the file "replaces duplicated text in briefs rather than adding to them".

Done looks like:

• [x] Each passage in src/crew/brief.ts, src/crew/conversation_brief.ts and CLAUDE.md that restates a principle in docs/PRINCIPLES.md is removed, or cut to a pointer. The pull lists each passage with what replaced it.
• [x] Brief size drops: a test asserts the builder brief is shorter than it is on main today, by a stated number.
• [x] The brief, conversation-brief and principles tests pass.

Files: src/crew/brief.ts, src/crew/conversation_brief.ts, src/crew/principles.ts, CLAUDE.md, test/principles.test.ts Risks: do not remove an instruction that docs/PRINCIPLES.md does not hold. Where in doubt, move it into the file within its length cap, or leave it.

Reviewed pull #8784 at its head: ticked Each passage in src/crew/brief.ts, src/crew/conversation_brief.ts and CLAUDE.md that restates a principle in docs/PRINCIPLES.md is removed, or cut to a pointer. The pull lists each passage with what replaced it.; Brief size drops: a test asserts the builder brief is shorter than it is on main today, by a stated number.; The brief, conversation-brief and principles tests pass..