chore(build): make xce-build.sh self-wrap into nix develop

meson/ninja/capnp/Go only exist inside the hermetic nix dev shell, but
xce-build.sh never entered it itself -- the /xce-build skill documented
wrapping every invocation in `nix develop ./src --command ...` by hand,
so a bare `bash src/tools/xce-build.sh ...` (run directly instead of
through the skill) failed opaquely with "meson: command not found". This
was a recurring, previously-logged gap that resurfaced again today mid
bug-1789 verification.

The script now checks IN_NIX_SHELL at the top and re-execs itself under
`nix develop` if unset, so every caller -- the skill, a bare invocation,
CI, a human at a terminal -- gets it automatically. A nested call (already
inside the shell) is a no-op past that check. README.md's daily-develop
section updated to match; the old "or use nix develop --command" example
is now redundant and removed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GLfB5uWRdmx2WiZSFC7Fm7

.wolf/cron-state.json

@@ -1,14 +1,7 @@
 {
-  "last_heartbeat": "2026-08-28T06:29:24.365Z",
+  "last_heartbeat": "2026-09-07T14:41:46.821Z",
   "engine_status": "running",
   "execution_log": [
-    {
-      "task_id": "project-suggestions",
-      "status": "failed",
-      "timestamp": "2026-05-21T13:48:25.789Z",
-      "duration_ms": 120850,
-      "error": "claude -p failed: spawnSync /bin/sh ETIMEDOUT"
-    },
     {
       "task_id": "anatomy-rescan",
       "status": "success",
@@ -620,6 +613,12 @@
       "status": "success",
       "timestamp": "2026-08-28T01:00:00.042Z",
       "duration_ms": 32
+    },
+    {
+      "task_id": "anatomy-rescan",
+      "status": "success",
+      "timestamp": "2026-09-07T13:00:00.252Z",
+      "duration_ms": 246
     }
   ],
   "dead_letter_queue": [],