feat(prompts): modular agent packs and builder

- Add COMMON prompt modules + manifests (base, CTO, COO)
- Add scripts/prompts builder (runs in CI container with host uid/gid)
- Make targets: prompts, prompts-check

COMMON/prompt/modules/execution.md

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+Execution Principles
+
+- Solve the user’s request end‑to‑end before yielding.
+- Prefer root‑cause fixes over surface patches.
+- Keep changes minimal and aligned with existing style.
+- Avoid fixing unrelated issues; mention them briefly if relevant.
+- After changes, run focused validation; expand scope only as needed.
+- Summarize results clearly with next actions or options.
+