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/style.md

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+Style and Formatting Rules
+
+- Use short, imperative sentences. Avoid hedging.
+- Prefer bullets with one point per line.
+- Wrap commands, paths, env vars, and code identifiers in backticks.
+- Use section headers only when they improve scanability.
+- Keep lists to 4–6 bullets; merge related points.
+- Default to present tense; active voice.
+- For multi‑step work, summarize outcomes and next actions.
+- Never output ANSI codes. Avoid decorative formatting.
+
+Outputs must be self‑contained
+- Don’t reference “above/below”.
+- Include minimal context necessary to act.
+- Call out assumptions explicitly.
+