orchestration: sprite task lease + durable wave-completion record

The wave-3 loss was caused by sprite auto-suspend, not nohup process-group
semantics. /.sprite/llm.txt: 'When idle, sprites pause automatically. Services and
sessions keep sprites alive.' Detached processes are on neither list, so setsid is
necessary but not sufficient.

tasks/wave-guard.sh holds a /v1/tasks lease (max 3600s, renewal is DELETE+POST since
re-POST returns 409), renews every 15 min while workers run, writes logs/WAVE<N>-DONE,
then releases the lease so the sprite can suspend rather than idle hot.

Also documents hazard #6 (pgrep -f / pkill -f matching the orchestrator's own shell)
and adds the wave-3 worker prompts and shared screen contract.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tasks/wave-sentinel.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# wave-sentinel.sh <sentinel-name> <task> [<task>...]
+# Detached watcher: waits for the named workers to stop, then writes a durable
+# completion record. Survives the orchestrator exiting, so a FRESH session can
+# learn a wave finished without having been present when it happened.
+set -u
+SENT="$1"; shift
+L="$HOME/bookshelf/logs"; OUT="$L/$SENT"
+while pgrep -f 'run-task\.sh|run-resume\.sh' >/dev/null; do sleep 30; done
+{
+  echo "=== $SENT written $(date -Is) ==="
+  echo "Workers finished. The orchestrator was NOT necessarily alive for this."
+  echo
+  for t in "$@"; do
+    echo "--- $t ---"
+    grep -hE 'SUCCESS|GIVING UP|WALL CLOCK|QUOTA' "$L/$t.state" 2>/dev/null | tail -3
+    if [ -s "$L/$t.json" ]; then
+      jq -r '"cost=$" + ((.total_cost_usd//0)|tostring) + " turns=" + ((.num_turns//0)|tostring)' "$L/$t.json" 2>/dev/null
+    else
+      echo "!! $t.json is 0 bytes -> worker was KILLED, not finished (hazard #3/#5)"
+    fi
+    echo
+  done
+  echo "NEXT: orchestrator must independently verify before accepting:"
+  echo "  cd ~/bookshelf && ./tasks/gw assembleDebug && ./tasks/gw testDebugUnitTest"
+  echo "  git status --porcelain   # boundary check: who touched what"
+} > "$OUT"