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

AI_CAPABILITIES_ROADMAP.md

@@ -1,5 +1,11 @@
 # Vibn AI Capability Roadmap
 
+> **⚠ See also:** [`AI_PATH_B_EXECUTION_PLAN.md`](./AI_PATH_B_EXECUTION_PLAN.md)
+> — proposed pivot to a Claude-Code-style persistent dev container per
+> project. Once approved, that doc supersedes any "code authoring" item
+> in this roadmap; this file remains the source of truth for
+> infrastructure primitives (P5.x, P6.x, P7.x).
+>
 > The ordered plan for closing the gap between what the Vibn agent can do
 > today and what it needs to do for a real customer to ship, operate, and
 > scale a SaaS through it.