bug: Settings → Trinity Prompt box should link to the recommended-prompt docs page #3206

Summary

The Settings → Trinity Prompt "How it works" box (fixed in #3195) gives the recommended-prompt guidance but doesn't link to the page that explains it, because that page wasn't on dev yet. It is now: docs/user-docs/agents/recommended-fleet-prompt.md (#3188).

Acceptance Criteria

• The "How it works" box in src/frontend/src/views/Settings.vue links to the recommended Trinity prompt docs page, using the same link pattern other Settings sections use for user docs
• The link opens the published docs page, not a repo path
• The existing trinityPromptCopy.spec.js covers the link
• Copy only; no behavior change

Follow-up to #3194 (its last acceptance item).