Slice 2: port 3452, nginx sandbox mount, AGENTS.md, docs, deploy script cleanup

- control-plane default listen addr is now :3452 (was :8080). An
  unusual port to avoid collisions on the VM.
- agent-micro and agent-gateway default SDP_CP_URL points at
  ws://localhost:3452/ws/agent. docker-compose.yml updates the
  control plane command, host port mapping, and agent -cp URLs.
- nginx/nginx.conf (the legacy root-mount reference) uses
  127.0.0.1:3452 for the upstream. nginx/sandbox.conf is the new
  deployment config: four location blocks for the /sandbox/credit-card
  mount — _next/static serves cached chunks, /api/ and /ws/ proxy
  to 127.0.0.1:3452, /sandbox/credit-card serves the static
  dashboard with try_files for SPA routing.
- scripts/patch-nginx.sh: deleted. The user configures nginx on 186
  by hand. scripts/deploy.sh no longer calls it.
- AGENTS.md: new file. Documents the build/lint/test commands
  (with the golang:1.24-alpine container — local Go can't fetch
  the toolchain), the wire protocol, the Slice-2 conventions
  (sdp-<repo> container naming, snapshot persistence,
  PreGitReset/AfterStart hooks), the repo-path gotcha, and the
  build-artifacts-in-git rationale.
- dashboard/out: now tracked in git, alongside bin/. The dashboard
  static export is scp'd to 186 on deploy; the VMs have no
  internet so they can't regenerate it. .gitignore comment
  explains this and warns against re-ignoring.
- README.md / REQUIREMENTS.md: status updated to 'Slice 2 done',
  per-feature checklist marked. Erangel repo path corrected to
  /var/www/html/erangel-ocean (was wrongly ~/SDP in earlier docs).

scripts/deploy.sh

@@ -4,8 +4,8 @@
 #   92 (micro):    ~/SDP/agent-micro
 #   186 (gateway): ~/SDP/control-plane, ~/SDP/agent-gateway, ~/SDP/dashboard
 #
-# On 186 we also splice the SDP location into nginx's existing default site
-# and reload. Run scripts/build.sh first.
+# Nginx is configured by hand on 186 (out of scope for this script).
+# Run scripts/build.sh first.
 
 set -euo pipefail
 cd "$(dirname "$0")/.."
@@ -47,10 +47,5 @@ $SCP_186 -r "$REPO_ROOT/dashboard/out/."  "$HOST_186:~/SDP/dashboard/"
 $SSH_186 "$HOST_186" "chmod +x ~/SDP/bin/control-plane ~/SDP/bin/agent-gateway"
 echo "    control-plane, agent-gateway, dashboard copied"
 
-# Patch nginx on 186
 echo
-echo "==> 186: patching nginx"
-"$REPO_ROOT/scripts/patch-nginx.sh"
-
-echo
-echo "done."
+echo "done. (configure nginx by hand on 186; see AGENTS.md for the location block.)"