feat: add Codex and OpenCode support

[aprprprr]

This PR adds first-class support for Codex and OpenCode.

What changed:

• added codex/AGENTS.md
• added codex/README.md
• added opencode/AGENTS.md
• added opencode/README.md
• updated the top-level README.md to list both tools in quick start, file counts, and tool descriptions

Why:

• Codex is currently only supported indirectly through the generic agents-md/AGENTS.md fallback
• OpenCode also benefits from a native package instead of relying on compatibility assumptions
• this repo already provides first-class packages for many other tools, so adding dedicated support improves discoverability and consistency

Verification:

• git diff --check
• copied both codex/AGENTS.md and opencode/AGENTS.md into temp project roots to confirm the package shape works as documented
• verified README counts and references are internally consistent

This is intentionally separate from PR #4 so the Claude Code bugfix review stays focused 🐾