fix(run): the FinalGuard nudge round can never replace a non-blank answer

Live regression (mort run d5ec39f4, 2026-07-15): a research run's page-cache
file false-positived the guard, and the nudge round's meta closer ("my
answer above stands as-is") REPLACED the real answer — hosts deliver
Output text plus separately-drained attachments, so the user received the
closer and never saw the answer.

New merge policy: the nudge round exists to make tool calls (queue the
delivery); its final text is discarded and recorded as a final_nudge_result
audit event, so a round that declined to queue stays observable without
reaching the user. The only exception is a blank original stop turn, where
the nudge round's text is the only answer there is. Messages/Usage still
merge (audit, PostRun, checkpoint continuity).

Additive on top of 007b753 (mort main pins that commit as a pseudo-version;
this branch must never be rewritten).

Co-Authored-By: Claude Fable 5 <[email protected]>

run/final_guard.go

@@ -20,10 +20,17 @@ import (
 // A non-empty nudge sends the loop back for ONE bounded extra round: the nudge
 // is appended to the SAME conversation as the next user turn and the agent
 // runs again with the same toolbox, capped at finalNudgeMaxSteps steps — enough
-// to deliver a stranded artifact (or correct the final text), not enough to
-// start a new project. The guard is consulted at most ONCE per run; the nudge
-// round's own stop turn is final. Multi-phase runs are not guarded (their
-// output contract is the phase pipeline's, not a delivery surface's).
+// to deliver a stranded artifact, not enough to start a new project. The guard
+// is consulted at most ONCE per run; the nudge round's own stop turn is final.
+// Multi-phase runs are not guarded (their output contract is the phase
+// pipeline's, not a delivery surface's).
+//
+// OUTPUT CONTRACT: the nudge round exists to make TOOL CALLS (queue the
+// delivery); its final TEXT is discarded — the run's Output stays the original
+// answer unless that answer was blank. A host's nudge message should say so
+// explicitly ("any text you write here is not shown to the user"), and must
+// not instruct the model to compose a corrected reply. Discarded nudge text is
+// recorded as a final_nudge_result audit event.
 //
 // The call runs on the run goroutine, inside the run's deadline, and is
 // panic-isolated: a guard panic is logged and treated as "" (the run's