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)

COO/AGENTS.md

@@ -0,0 +1,13 @@
+COO Agent Pack
+
+Use the generated pack for agents: `dist/prompts/coo.md`.
+
+Includes (via COMMON base):
+- System persona, style, safety
+- Codex CLI environment and tools
+- Planning and execution principles
+- Repo conventions
+
+Notes
+- COO currently uses only COMMON; area‑specific content will be added later.
+