feat(codex): migrate newsletter workflows to skills

docs/multi-tool-usage.md

@@ -8,7 +8,7 @@ This repo is usable from three AI coding tools off **one shared engine**. The ne
 |------|------------------|---------------------------|-------|--------------------|
 | **Claude Code** | `CLAUDE.md` → imports `AGENTS.md` | Native skill auto-dispatch from `.claude/skills/` | None (works as-is) | Paste a URL; `mt-add-url` auto-runs |
 | **OpenCode** | `AGENTS.md` (auto-read) | `.claude/skills/` auto-discovered via the `skill` tool, governed by `opencode.json` | None beyond `opencode.json` (committed) | Ask to add a URL; pick/`skill` `mt-add-url` |
-| **Codex** | `AGENTS.md` (auto-read) | Custom prompts copied to `~/.codex/prompts/` | `bash codex/install.sh` (Linux/macOS) or `pwsh codex/install.ps1` (Windows) | `/prompts:mt-add-url <url>` |
+| **Codex** | `AGENTS.md` (auto-read) | Repository skills discovered from `.agents/skills/` | None (works as-is) | Ask to add a URL or invoke `$mt-add-url` |
 
 **Shared engine:** all three call `node scripts/newsletter/*.js` from repo root — no per-tool script copies.
 
@@ -16,14 +16,14 @@ This repo is usable from three AI coding tools off **one shared engine**. The ne
 
 - **Claude Code** — `CLAUDE.md` is a thin file that imports `AGENTS.md` (`@AGENTS.md`); all instruction content lives in `AGENTS.md`. Permissions in `.claude/settings.json`.
 - **OpenCode** — reads `.claude/skills/<name>/SKILL.md` natively (multi-location search up the worktree), so the same `mt-*` skills work with zero duplication. `opencode.json` mirrors the Claude allow-list intent (node/git/hugo/edit). No MCP block — relies on OpenCode's built-in web fetch/search. Restart OpenCode after editing `opencode.json` (config is not hot-reloaded).
-- **Codex** — no skill auto-dispatch; loads prompts only from `~/.codex/prompts/` (project-level `.codex/prompts` is not yet shipped). The installer **copies** prompt sources there (not symlinks — Windows symlinks need admin/Developer Mode). **Re-run the installer after editing any `codex/prompts/*.md`.** Even without installing, Codex can read `AGENTS.md` and call the scripts directly.
+- **Codex** — reads repository skills from `.agents/skills/` and can invoke them explicitly (`$mt-add-url`) or implicitly from their descriptions. The checked-in Codex skills delegate to the canonical workflow definitions in `.claude/skills/`, so workflow behavior stays shared without an install or sync step.
 
 ## Teardown — pick one, remove the rest
 
 Once you commit to one tool, delete the others' config:
 
-- **Keep Claude Code only** → remove `opencode.json` and `codex/`; uninstall the prompts with `rm ~/.codex/prompts/mt-*.md`. Keep `.claude/`.
-- **Keep OpenCode only** → remove `.claude/settings*.json` and `codex/` (+ `rm ~/.codex/prompts/mt-*.md`). **Keep `.claude/skills/`** — OpenCode reads it. If you want to drop the `.claude/` dir entirely, first move skills to `.opencode/skills/` (scripts are path-neutral in `scripts/newsletter/`, so the move breaks nothing). Optionally make `AGENTS.md` the only instruction file (delete `CLAUDE.md`).
-- **Keep Codex only** → remove `opencode.json` and `.claude/`; the workflow survives as `codex/prompts/` + `AGENTS.md`. Keep `scripts/newsletter/` and re-run `codex/install.sh`.
+- **Keep Claude Code only** → remove `opencode.json` and `.agents/`. Keep `.claude/`.
+- **Keep OpenCode only** → remove `.claude/settings*.json` and `.agents/`. **Keep `.claude/skills/`** — OpenCode reads it. If you want to drop the `.claude/` dir entirely, first move skills to `.opencode/skills/` (scripts are path-neutral in `scripts/newsletter/`, so the move breaks nothing). Optionally make `AGENTS.md` the only instruction file (delete `CLAUDE.md`).
+- **Keep Codex only** → keep `.agents/skills/`, `AGENTS.md`, `scripts/newsletter/`, and the canonical `.claude/skills/` files referenced by the Codex skills. You may remove Claude-specific settings and `opencode.json`.
 
-In every case: **keep `scripts/newsletter/` and `AGENTS.md`** — they are the shared engine and instructions.
+In every case: **keep `scripts/newsletter/`, `.claude/skills/`, and `AGENTS.md`** — they are the shared engine, canonical workflows, and instructions.