User guide: link a thread to Build work #109

Part of #78. Fixes the blocking finding from #78's Spec review: acceptance 8 is missing.

Scope

Add a short section to the user guide (docs/user/) on linking a thread to a YardSmith HQ Build Spec or Ticket. Cover: the project needs HQ configured; how to link, change and unlink (from Link Build item in the thread menu or command palette, and from the segment's menu); that agents can link their own threads; that Build links and #<number> (seven or more digits) in chat become live chips; and what you see when Build can't be reached. Follow the Documentation rules in AGENTS.md.

Classification

Non-ui (documentation).

Acceptance

#78 acceptance 8.