#2945 stagecraft: the Linear adapter assigns the issue when work starts

The Linear tracker adapter never assigns an issue. #2801 made publish assign the ticket when a work item starts, but only through a tracker's assign capability and an assignee mapping, and the Linear adapter has neither (capabilities: {}; the code says "absent where no login maps to a user yet (Linear)"). A live smoke test against a real Linear workspace confirmed it: an issue driven from start to In Progress stays unassigned, with no warning.

Proposed:

• Give the Linear adapter the assign capability: read the issue's assignee, write nothing when it is already the user, otherwise set assigneeId. A Linear issue has one assignee, so assign replaces whoever is there and reports them in dropped.
• Whom publish assigns on Linear: the API key's owner (Linear's viewer). Every Linear write already acts as that user, and no swamp login maps to a Linear user.
• A manual assign method on the Linear model (issue, optional user, defaulting to the key's owner), as the Lab has.
• fetch_issue records the issue's assignee.
• publish's assign log names the user by a display name, not a raw id.
• The Linear fake learns users, the viewer and the single assignee; the conformance suite covers Linear's assign; README and REFERENCE describe it for Linear.