Link a thread to a Build item and show it in the header #79

Part of #78. That Spec's Design and Settled constraints are the contract; this Ticket covers the link, the server's compact read and the header.

Scope

• Link. Add the thread's Build link to thread metadata: Build item id, number and HQ origin. Set, replace and clear it through one service method, reached by:

  • t3_thread_update, which can also act on a delegated thread;
  • the segment menu's Change and Unlink;
  • Link Build item in the web header's thread menu, the command palette and mobile's thread menu.

  Change and Link take a Build URL or a number.

• Build read service.

  • Exchange and renew the server's observer token from Inject a per-agent YardSmith HQ MCP connection for each T3 thread #74's stored delegation, and end it when no project uses the connection. The exchange takes role observer, harness t3, no model, and agentRef set to the bare <environmentId>; HQ refuses a / in it with 400 agent_ref_invalid (pechhe/yardsmith#2485, landed as 319753ac).
  • Resolve a number with hq_build_read.
  • Serve a compact view (title, number, kind, status, holds, waits) keyed by HQ connection and item.
  • Re-read item and waits once a minute only while a client shows it.

• Header segment on web, as designed (glyph, title, status dot, ⏸ Held and ⛓ Blocked from Build's waits). Clicking it opens /build/<number> in HQ. On mobile, the item is the header subtitle and tapping it opens HQ; Spec and Ticket cards for the thread's Build item #80 adds the sheet.

• Unavailable and cold states as the Spec states. Projects without HQ show nothing.

Classification

Mixed. The server, contracts, MCP action and menu entries are non-ui wiring. The breadcrumb segment and the mobile subtitle are UI design, following the agreed prototype.

Acceptance

#78 acceptance 1, 5 (compact read and its cadence) and 6, plus the header-overlay parts of 2 (⏸ Held, and ⛓ Blocked for Ticket and Spec blockers), on web and mobile. The tests use a controlled clock and a fake HQ MCP, and include a revoked observer token.