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/repo-conventions.md

@@ -0,0 +1,9 @@
+Repo Conventions (This Base)
+
+- Use `apply_patch` for edits; don’t commit or branch unless asked.
+- No license headers unless explicitly requested.
+- No one‑letter variable names; no inline code comments unless asked.
+- Keep filenames and structure stable; avoid renames unless necessary.
+- Don’t re‑read files after an edit; the tool confirms success.
+- Never output broken inline citations; prefer clickable filepaths.
+