docs(agents): add AGENTS.md and area packs

- Add CTO/COO AGENTS.md pointing to dist prompt packs
- Add CODEOWNERS for COMMON/prompt and areas
- Add prompt_build.py used by scripts/prompts (containerized)

CTO/AGENTS.md

@@ -0,0 +1,13 @@
+CTO Agent Pack
+
+Use the generated pack for agents: `dist/prompts/cto.md`.
+
+Includes (via COMMON base):
+- System persona, style, safety
+- Codex CLI environment and tools
+- Planning and execution principles
+- Repo conventions
+
+Notes
+- Keep CTO‑specific additions minimal; prefer COMMON as source of truth.
+