Extract newsletter scripts to a neutral scripts/newsletter/ engine and add
side-by-side support for all three AI coding tools off one source of truth.
- Move 9 scripts + substack config from .claude/skills/**/scripts to
scripts/newsletter/ (history preserved); fix PROJECT_ROOT depth and
cross-require/config paths; repoint all SKILL.md invocations
- Add canonical AGENTS.md; reduce CLAUDE.md to an @AGENTS.md import
- Add opencode.json (permissions, no MCP); skills auto-discovered in place
- Add codex/prompts/*.md (6 prompts) + copy installers (install.sh/.ps1)
- Add docs/multi-tool-usage.md (setup, per-tool invocation, teardown) + README pointer
