feat(unic-archon-dlc): /build's loop has no deterministic exit for "blocked, needs a human" #656

Observed

Two runs of unic-dlc-build (unic-archon-dlc 0.29.0, Archon 0.11.1, Windows 11), Consumer Becklyn-Studios/pr-review-agent.

Run 34f986ef: the loop spun until killed.

• Slice review-instructions could not take RED: its eval (pnpm eval review-core) had no model gateway inside the Archon worktree (MODEL_CALL_FAILED).
• 16 consecutive iterations each spawned a fresh session, re-ran the eval, reported BLOCKED, appended a "RERUN n" note to build-state.json, and did not emit COMPLETE.
• From iteration 14 on, the iterations themselves recommended stopping the loop.
• The loop ended only when the operator pressed Ctrl+C.

Run 9ff2d6da (resumed): the loop exited by accident.

• Slice acceptance-run is live-only (test_command_planned: true; it needs a deploy and a real PR on a sandbox repository).
• 14 identical BLOCKED iterations followed.
• Then: Loop node 'run-build' completed after 14 iterations, with acceptance-run still pending.
• The final iteration's message ends with "I have not emitted ``". The word appears to have been consumed by the legacy prose until: COMPLETE match.
• Earlier iterations with similar wording ("Not emitted: COMPLETE", "I did not emit COMPLETE") did not stop the loop.

At startup Archon warns about this exact node:

Node 'run-build': the prose 'loop.until' completion signal is deprecated. Declare 'loop.until_bash' (deterministic check) or 'loop.until_field' (a declared boolean in output_format) instead

Why it matters

• Cost. A blocked slice spends up to max_iterations (60) fresh sessions producing the same BLOCKED report.
• Unpredictable exit. Whether the loop exits depends on how the agent phrases its refusal.
• Early exit. The same accidental match could end the loop with slices still unbuilt and unblocked. For example, "I won't emit COMPLETE until checkout is green" in slice 3 would send 15 unbuilt slices to review.

The escape hatch itself is right: a blocked loop should exit to the report and the gate. It just needs to be deliberate.

Suggested

• Migrate run-build to loop.until_field, with a declared status: complete | blocked (plus the blocking slice and reason) in output_format, as Archon's deprecation warning asks.
• Route blocked to report and the gate, naming the blocker, without opening a PR.
• Optionally, have /tickets keep live-only slices (test_command_planned plus outward actions such as deploys) out of the build baton entirely, and relabel their tracker items ready-for-human.

Not decided

• Whether a blocked run should still run implement-review-precheck and goals-check on the slices that did go green.
• Whether a repeat-blocked threshold (for example, N identical BLOCKED iterations) should also exit, as a guard for agents that fail to set the field.