fix(TASK-020): surface step-runner LLM-fault captures where the fallback lands

The three fallback-capture keys (orient_failure, sweep_failure,
handoff_failure) were write-only dead state: captured on LLM-node failure
but interpolated nowhere, so end_failure rendered an empty STEP_FAILED
shell and write_handoff carried no trace a dead sweep.

- new crash-proof script node note_llm_fault distills orient/handoff
  'LLM node...' failure text into fault_note (prefix-anchored; empty when
  neither key holds a fault); orient and write_handoff fallbacks now route
  through it before end_failure
- end_failure renders 'Pipeline fault (if any): {{fault_note}}' (sentinel
  and all existing fields preserved; fault_note defaulted in initial_state)
- write_handoff prompt interpolates {{sweep_failure}} with a
  PIPELINE-FAULT VISIBILITY instruction mirroring the review_report
  treatment (fault -> Deviations entry + step_summary; success/empty
  ignored)
- tests: note_llm_fault fault/clean/crash/truncation paths via the
  GRAPH_STATE harness; wiring test pins the retargeted fallbacks,
  note_llm_fault routing, {{fault_note}} in end_failure, and the
  sweep-fault prompt/instruction anchors; warning baseline unchanged

assets/agents/step-runner/graph.yaml

@@ -89,6 +89,7 @@ initial_state:
   handoff_attempts: 0
   handoff_fix: ''
   handoff_failure: ''
+  fault_note: ''
   step_summary: ''
 
 start: resolve_step
@@ -196,7 +197,7 @@ nodes:
         ]
     state_updates:
       orient_failure: '{{output}}'
-    fallback: end_failure
+    fallback: note_llm_fault
     next: route_staleness
 
   route_staleness:
@@ -474,6 +475,16 @@ nodes:
       did not run - <failure text>", repeat it under Notes for next step,
       and carry it into `step_summary` so the user sees it at the approval
       gate.
+
+      The same rule covers the "Edge-case sweep result" below: text starting
+      with "LLM node" means the edge-case sweep DID NOT RUN - the graph
+      routed here through the sweep's failure fallback. Never present it as
+      sweep findings. Add a Deviations entry "PIPELINE-FAULT: edge-case
+      sweep did not run - <failure text>" and carry it into `step_summary`
+      so the user sees it at the approval gate. If it holds successful
+      structured output or is empty, ignore it - the sweep's real results
+      already flow through the "Edge cases discovered" / "Downstream plan
+      updates" sections above.
     prompt: |
       ## Step
       {{step_number}} ({{step_title}}) - plan at {{step_plan_path}}
@@ -497,6 +508,9 @@ nodes:
       ## Downstream plan updates
       {{downstream_updates}}
 
+      ## Edge-case sweep result (fault detection)
+      {{sweep_failure}}
+
       ## Independent review report (if any)
       {{review_report}}
 
@@ -527,9 +541,22 @@ nodes:
       required: [step_summary]
     state_updates:
       handoff_failure: '{{output}}'
-    fallback: end_failure
+    fallback: note_llm_fault
     next: check_handoff
 
+  note_llm_fault:
+    id: note_llm_fault
+    type: script
+    description: |
+      Make LLM-node fallback captures visible: when orient or write_handoff
+      failed (their capture keys hold the engine's "LLM node..." failure
+      text), distill it into fault_note so end_failure can render WHY the
+      step died instead of an empty STEP_FAILED shell.
+    script: scripts/note_llm_fault.sh
+    timeout: 5
+    fallback: end_failure
+    next: end_failure
+
   check_handoff:
     id: check_handoff
     type: script
@@ -627,6 +654,7 @@ nodes:
       Step: {{step_number}} ({{step_title}})
       Fix attempts: {{fix_attempts}}/{{max_fix_attempts}}
       Blocking reason (if resolution failed): {{blocking_reason}}
+      Pipeline fault (if any): {{fault_note}}
 
       Coder result:
       {{coder_result}}

assets/agents/step-runner/scripts/note_llm_fault.sh

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ -n "${GRAPH_STATE_FILE:-}" ]]; then
+  state=$(cat "$GRAPH_STATE_FILE")
+elif [[ -n "${GRAPH_STATE:-}" ]]; then
+  state="$GRAPH_STATE"
+else
+  state='{}'
+fi
+
+orient_failure=$(echo "$state" | jq -r '.orient_failure // ""')
+handoff_failure=$(echo "$state" | jq -r '.handoff_failure // ""')
+
+# Engine failure text is always prefix-anchored "LLM node" (llm.rs); on
+# success these keys hold the node's structured JSON output, which never
+# starts with that prefix.
+if [[ "$orient_failure" == "LLM node"* ]]; then
+  fault_note="PIPELINE-FAULT: orient stage failed — ${orient_failure:0:300}"
+elif [[ "$handoff_failure" == "LLM node"* ]]; then
+  fault_note="PIPELINE-FAULT: handoff stage failed — ${handoff_failure:0:300}"
+else
+  fault_note=""
+fi
+
+jq -nc --arg note "$fault_note" '{"fault_note": $note}'