chore(13): execute plans sequentially — this box thrashes on parallel Rust builds

4 cores, load 35, 15G of 23G swap in use, rustc at 8.3G RSS while a live
node (bitcoind/electrumx/lnd) shares the machine. Two concurrent cargo
builds in separate worktrees (no shared target dir) made wave 1 crawl for
over an hour with zero commits. Wave 2 has four plans, so this would have
gotten worse before it got better.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.planning/config.json

@@ -0,0 +1,6 @@
+{
+  "workflow": {
+    "_auto_chain_active": false
+  },
+  "parallelization": false
+}