feat: add Codex agents/openai.yaml metadata to every skill

Adds an `agents/openai.yaml` beside each `SKILL.md` so the skills work in
Codex as well as Claude Code, without generated copies:

- `interface.display_name` + `interface.short_description` for the Codex
  skill picker, hand-written for all 39 skills.
- `policy.allow_implicit_invocation: false` on the 22 user-invoked skills —
  the Codex analog of `disable-model-invocation: true`, so Codex excludes
  them from implicit invocation while explicit `$skill` still works.
- Document the dual-harness invocation model in `.agents/invocation.md`,
  `CLAUDE.md`, and the promoted-bucket READMEs.
- Add `AGENTS.md` as a symlink to `CLAUDE.md` so Codex reads the same
  instructions; note Codex as a `link-skills.sh` install target.

Slimmed-down rework of the approach prototyped in #522: keeps the essential
cross-harness metadata, drops the Ruby validator, the runtime-detector test,
per-skill `default_prompt`s, and the unrelated promotion changes.

Co-authored-by: gabimoncha <gabimoncha@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/link-skills.sh

@@ -8,7 +8,7 @@ set -euo pipefail
 # Links all skills in the repository into the local skill directories used by
 # each agent harness:
 #   - ~/.claude/skills  — Claude Code
-#   - ~/.agents/skills  — pi and other Agent-Skills-standard harnesses
+#   - ~/.agents/skills  — Codex and other Agent Skills-compatible harnesses
 # Each entry is a symlink into this repo, so a `git pull` is all that's needed
 # to keep installed skills up to date.