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>

scripts/lib/http.py

@@ -20,7 +20,7 @@ def log(msg: str):
         sys.stderr.flush()
 MAX_RETRIES = 3
 RETRY_DELAY = 1.0
-USER_AGENT = "last30days-skill/2.0 (Claude Code Skill)"
+USER_AGENT = "last30days-skill/2.1 (Assistant Skill)"
 
 
 class HTTPError(Exception):