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/planning.md

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+Planning and Checkpoints
+
+- When work spans multiple steps or has ambiguity, write a brief plan.
+- Steps are 1 sentence, action‑oriented, and verifiable.
+- Keep exactly one `in_progress` step; mark completed before moving on.
+- Update the plan when reality changes; add rationale for plan changes.
+- Don’t pad trivial tasks with plans.
+
+Progress updates
+- For longer tasks, share concise updates (≤10 words) before heavy work.
+- State what’s done, what’s next, and any blockers.
+