fix(#3795): read the interrupted agent id before clearing the stale marker (#4006)

* test(#3795): the interrupted-agent read must precede the stale-id clear

* fix(#3795): read the interrupted agent id before clearing the stale marker

execute-plan's init_agent_tracking step ran `rm -f
.planning/current-agent-id.txt` BEFORE the existence check that read it,
so the interrupted-agent branch and the Task resume prompt it exists to
offer were unreachable (#3795) — a kill -9 mid-executor left the file,
and the next run deleted it before looking. The read now precedes the
clear; fresh-run semantics (no stale id leaking into the new spawn) are
preserved. A structural guard pins the order.

Emitted-Drift-Ack-Growth: execute-plan.md — #3795: +bytes from reordering the interrupted-agent read before the rm plus the explaining comment

* chore(#3795): changeset fragment (pr number backfilled after PR creation)

* chore(#3795): backfill changeset PR number (4006)

---------

Co-authored-by: sim <sim@local>

gsd-core/workflows/execute-plan.md

@@ -155,11 +155,14 @@ Fresh context per subagent preserves peak quality. Main context stays lean.
 if [ ! -f .planning/agent-history.json ]; then
   echo '{"version":"1.0","max_entries":50,"entries":[]}' > .planning/agent-history.json
 fi
-rm -f .planning/current-agent-id.txt
+# #3795: read a SURVIVING id before clearing it — the clear used to run
+# first, so the check below was always false and the resume prompt at this
+# step's tail was never offered.
 if [ -f .planning/current-agent-id.txt ]; then
   INTERRUPTED_ID=$(cat .planning/current-agent-id.txt)
   echo "Found interrupted agent: $INTERRUPTED_ID"
 fi
+rm -f .planning/current-agent-id.txt
 ```
 
 If interrupted: ask user to resume (Task `resume` parameter) or start fresh.