feat: Add Codex CLI compatibility

- Add agents/openai.yaml for Codex skill discovery
- Make SKILL.md script path portable (repo, Claude, Codex, agents dirs)
- Platform-neutral output text ("assistant" instead of "Claude")
- Sandbox-friendly cache/output dirs with env var overrides and tempdir fallback
- Add Codex installation docs to README

Inspired by PR #24 (el-analista) and PR #5 (jblwilliams).
Zero impact on existing Claude Code behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -53,6 +53,16 @@ node ~/.claude/skills/last30days/scripts/lib/vendor/bird-search/bird-search.mjs
 
 **Requirements:** Node.js 22+ (for the vendored Twitter GraphQL client).
 
+### Codex CLI
+
+This skill also works in OpenAI Codex CLI. Install to the Codex skills directory instead:
+
+```bash
+git clone https://github.com/mvanhorn/last30days-skill.git ~/.agents/skills/last30days
+```
+
+Same SKILL.md, same Python engine, same scripts. The `agents/openai.yaml` provides Codex-specific discovery metadata. Invoke with `$last30days` or through the `/skills` menu.
+
 ## Usage
 
 ```