bug(unic-archon-dlc): a green /build that cannot open its pull request ends completed #666

Problem

After #439, a /build run with a pass verdict reaches open-pr, which opens the pull request. open-pr is a prompt node. When it cannot open the pull request, it prints BLOCKED: … with the tool's error, but the node still ends green. A prompt node cannot fail by reporting.

• In afk, build-pr-gate is skipped by its own when, and the run ends completed with no pull request.
• In hitl, build-pr-gate asks a human to review a pull request that does not exist, and an approval ends the run completed.

Causes include a tracker contract that names a repository other than origin (a fork setup), a tracker tool or permission error, and a missing docs/agents/issue-tracker.md. The failure shows up only later, when /qa cancels at guard-no-open-pr (#652), away from its cause.

Found by review round 2 of PR #665, outside that ticket's scope. The class predates that pull request.

Acceptance criteria

• A /build run whose open-pr opened no pull request does not end completed, in afk or in hitl.
• The run's last message says that the verdict was pass, that no pull request was opened, why, and on which branch and commit the work is.
• build-pr-gate never asks for a review of a pull request that does not exist.
• docs/boxes/unic-dlc-build.md describes the case and its message.
• A run that opens its pull request behaves as it does today.