Add agmsg skill: cross-agent messaging via SQLite for CLI agents

[fujibee]

Summary

Adds agmsg as a curated skill under skills/.curated/agmsg/. agmsg is a small shell-based tool that lets CLI AI agents — Codex, Claude Code, Gemini CLI, GitHub Copilot CLI, Antigravity, OpenCode, and others — send messages to each other through a shared local SQLite database. No daemon, no network, no Python.

Canonical project: https://github.com/fujibee/agmsg
Homepage: https://agmsg.cc

What's in the PR

• skills/.curated/agmsg/SKILL.md — frontmatter (name + description) and the workflow. Describes when to use agmsg (delegating tasks between agents, coordinating multi-agent work, async handoffs) and walks through the daily flow: identity check → inbox → send / history / team list, plus the optional actas / drop flow for multi-role projects.
• skills/.curated/agmsg/LICENSE.txt — MIT.

Why this fits a curated catalog entry

• Single, focused capability (cross-agent messaging) with a clear "when to use" trigger.
• No new dependencies beyond bash and sqlite3 — both available by default on macOS and most Linux distributions.
• Codex-relevant: the skill already ships a Codex-tuned delivery mode (turn, Stop hook fires check-inbox.sh) and the SKILL.md is written from the Codex perspective.
• Cross-agent by design — Codex sessions can message Claude Code / Gemini CLI / Copilot CLI sessions on the same machine, which is otherwise hard to set up.

Install path

After landing here, Codex CLI users can install via:

$skill-installer install agmsg

The skill body links back to the canonical install script (bash <(curl -fsSL .../setup.sh)) which sets up the SQLite store and hook plumbing under ~/.agents/skills/agmsg/.

Test plan

• SKILL.md frontmatter has name and description (per the repo convention).
• LICENSE.txt present (MIT).
• Skill is read-only by intent and instructs the agent to "always go through the scripts" (no direct edits to team/db state).
• Reviewer: please flag anything that diverges from .curated/ conventions and I'll adjust.

Cross-links

• Project: https://github.com/fujibee/agmsg
• Tracked on our side: Submit agmsg to openai/skills Codex official catalog fujibee/agmsg#78 (epic Epic: Preemptive name registration for 'agmsg' across plugin/skill ecosystems fujibee/agmsg#75 — preemptive name registration in plugin / skill marketplaces).