feat(nh3-extdev): install pi (earendil-works) + wire /opt/externs client agents to GLM 5.2

- user-level Node v22.23.0 LTS (static tarball, checksum-verified) + pi 0.79.7
  installed -g into the user prefix (box is sudo-less: no root/apt/docker)
- every /opt/externs/<client> wired to GLM 5.2 via the litellm gateway with an
  isolated PI_CODING_AGENT_DIR + scoped per-client key (models.json/settings.json
  + run-pi.sh launcher); replaces the scaffold's incorrect config.example guess
- playbooks/install-pi-nh3-extdev.yaml: idempotent reproduce / add-client / upgrade
  (validated clean: 4 steps skipped, live gbcnc->GLM 5.2 round-trip OK)
- README: settled role + per-client workspace layout; refresh system-details

servers/nh3-extdev/system-details.txt

@@ -2,8 +2,8 @@
 ===== HOST =====
 
 Hostname:   nh3-extdev
-Date:       2026-06-17T14:55:16-07:00
-Uptime:     up 16 minutes
+Date:       2026-06-18T14:04:13-07:00
+Uptime:     up 23 hours, 25 minutes
 OS:         Debian GNU/Linux 13 (trixie)
 Kernel:     6.12.90+deb13.1-amd64
 Arch:       x86_64
@@ -13,7 +13,7 @@ Arch:       x86_64
 CPU cores:  8
 CPU model:  QEMU Virtual CPU version 2.5+
 MemTotal:   7.8 GB
-MemAvailable: 7.4 GB
+MemAvailable: 7.3 GB
 
 ===== GPUS =====
 
@@ -22,7 +22,7 @@ nvidia-smi not present (no NVIDIA GPUs or driver not installed)
 ===== FILESYSTEMS (df) =====
 
 Filesystem      Size  Used Avail Use% Mounted on
-/dev/sda1       250G  3.2G  235G   2% /
+/dev/sda1       250G  3.6G  234G   2% /
 
 ===== PERSISTENT MOUNTS (/etc/fstab, non-comment) =====
 
@@ -32,10 +32,11 @@ UUID=bf3fd6d0-4537-4626-bdcc-551c5603ab98 none            swap    sw
 
 ===== TARGETED DATA PATHS =====
 
-/opt  (total: 4.0K)
-  total 8
-  drwxr-xr-x  2 root root 4096 2024-05-30 19:51 .
-  drwxr-xr-x 18 root root 4096 2026-06-17 14:37 ..
+/opt  (total: 1.5M)
+  total 12
+  drwxr-xr-x   3 root      root    4096 2026-06-17 15:03 .
+  drwxr-xr-x  18 root      root    4096 2026-06-17 14:37 ..
+  drwxrws---+  5 infra-ops externs 4096 2026-06-17 15:13 externs
 
 /srv  (total: 4.0K)
   total 8