docs(path-b): execution plan + vibn-dev image scaffold

- AI_PATH_B_EXECUTION_PLAN.md: add 3 safety nets (auto-push, kill
  switch, hard tool removal), tighten 4 risks (network policy week 1,
  HMR spike day 1, lean image + lazy mise, random preview suffix).
- AI_CAPABILITIES_ROADMAP.md: pointer note already in place.
- vibn-dev/Dockerfile + supervisord.conf + mise.default.toml + README:
  scaffold for the per-project dev container image. Ubuntu 24.04 +
  git + ripgrep + python3 + mise. Toolchains lazy-install on first
  `mise install`. Container runs as uid 1000 vibn (sudo available).

Frontend wiring lives in vibn-frontend (separate commit).

Made-with: Cursor

vibn-dev/supervisord.conf

@@ -0,0 +1,9 @@
+; supervisord config for vibn-dev. Currently a no-op — the container
+; runs `tail -f /dev/null` as PID 1 (via tini) and dev servers are
+; launched ad-hoc through `docker exec` + nohup once dev_server.start
+; ships in week 2. Reserved here so the conf path exists.
+[supervisord]
+nodaemon=true
+user=vibn
+logfile=/var/log/vibn-dev/supervisord.log
+pidfile=/tmp/supervisord.pid