feat: MergeFailureFinal → Coding via operator FixupRequested

MergeFailureFinal was unreachable from move_story: the only transitions
out were Freeze (→ Frozen) and a self-loop on MergemasterAttempted, so
once mergemaster exhausted its 3-retry budget the only way to get a
story coding again was to delete + recreate it.

The respawn budget is a mergemaster bookkeeping detail, not a hard
ceiling. A human operator inspecting a Final story can reasonably
decide the gate failure is fixable, so this adds the same
FixupRequested → Coding edge that already exists for plain
MergeFailure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/agents/lifecycle.rs

@@ -411,6 +411,9 @@ fn map_stage_move_to_event(
         (Stage::Merge { .. }, "current") => Ok(PipelineEvent::MergeAborted),
         // Story 971: send MergeFailure story back to Coding so a coder can fix it.
         (Stage::MergeFailure { .. }, "current") => Ok(PipelineEvent::FixupRequested),
+        // Operator override on the exhausted-respawn terminal state: still
+        // a coder fixup, but reached via the budget-exhausted path.
+        (Stage::MergeFailureFinal { .. }, "current") => Ok(PipelineEvent::FixupRequested),
         // Story 972: send MergeFailure story back to Qa for a QA agent to re-review.
         (Stage::MergeFailure { .. }, "qa") => Ok(PipelineEvent::ReQueuedForQa),
         // Story 974: reopen a Done story for a post-merge hotfix.

server/src/pipeline_state/transition.rs

@@ -307,6 +307,15 @@ pub fn transition(state: Stage, event: PipelineEvent) -> Result<Stage, Transitio
         // ── FixupRequested: MergeFailure → Coding (coder fixup) ────────
         (MergeFailure { .. }, FixupRequested) => Ok(Coding),
 
+        // ── FixupRequested: MergeFailureFinal → Coding (operator override)
+        //
+        // The exhausted-respawn-budget terminal state is not actually
+        // terminal as far as the operator is concerned; a human can decide
+        // the gate failure is fixable and send the story back for another
+        // coder attempt.  The budget counter is a mergemaster bookkeeping
+        // detail, not a hard ceiling.
+        (MergeFailureFinal { .. }, FixupRequested) => Ok(Coding),
+
         // ── ReQueuedForQa: MergeFailure → Qa (re-review) ────────────────
         (MergeFailure { .. }, ReQueuedForQa) => Ok(Qa),