Submit agmsg to openai/skills Codex official catalog

[fujibee]

Part of epic #75.

Goal

Add `skills/agmsg/` to openai/skills (Codex official catalog) via PR so Codex CLI users can install it as a built-in curated skill.

Once landed, users can install via:
```
$skill-installer install agmsg
```
or via the official skills directory in Codex CLI's skill manager.

Pre-submission

• Read `contributing.md` — the file exists at the repo root and is the canonical submission guide.
• Skill structure expected: `skills/agmsg/SKILL.md` (folder name = skill name) with YAML frontmatter (`name`, `description`) and a workflow body.
• The directory layout has three buckets: `.system` (auto-installed), `.curated` (PR-reviewed), `.experimental` (less-reviewed). agmsg should likely land under `.curated`.

What this PR contains

• New file: `skills/.curated/agmsg/SKILL.md` — adapted from our existing `templates/cmd.codex.md`, with the YAML frontmatter aligned to openai/skills conventions.
• Possibly a `LICENSE.txt` in the skill folder (per-skill licensing convention).
• A short `README.md` in the skill folder linking back to https://github.com/fujibee/agmsg and https://agmsg.cc.

Pre-PR checklist

• Read upstream `contributing.md`.
• Skill folder structure follows their conventions (`.curated//`).
• SKILL.md frontmatter has `name` and `description` at minimum.
• LICENSE.txt placed in the skill folder (MIT or matching upstream).
• Link back to the canonical repo and homepage.

Acceptance

• PR opened against `openai/skills`.
• Pipeline checks pass.
• PR merged.
• Verified install via `$skill-installer install agmsg` works end-to-end in a fresh Codex CLI session.

Cross-links

• Epic: Epic: Preemptive name registration for 'agmsg' across plugin/skill ecosystems #75
• Upstream: https://github.com/openai/skills

[fujibee]

Upstream PR opened: openai/skills#471 — adds skills/.curated/agmsg/ with SKILL.md + LICENSE.txt. Awaiting review/merge from openai/skills maintainers.