Add Codex support: single Codex skill, AGENTS.md, and installation/docs

[pcatattacks]

Motivation

• Make the Solopreneur plugin usable from OpenAI Codex by providing a native Codex skill entrypoint and repository-level guidance.
• Avoid duplicating workflow content in Codex by routing from a thin Codex skill into the existing shared skills/*/SKILL.md and agents/*.md files.
• Provide clear installation and usage instructions for Codex while keeping Claude Code plugin behavior unchanged.

Description

• Add a Codex-native routing skill at .agents/skills/solopreneur/using-solopreneur/SKILL.md that selects and delegates to the shared workflow files.
• Add repository-level Codex guidance in AGENTS.md to act as guardrails and document how Codex should use the existing workflows and role briefs.
• Add Codex install and usage documentation in .codex/INSTALL.md, docs/README.codex.md, and a small redirect docs/codex.md, and update README.md with Codex compatibility notes and examples.
• Keep all changes additive and documentation-focused so existing Claude plugin packaging, hooks, and settings remain untouched.

Testing

• No automated tests were added or modified for this documentation-and-wrapper change.
• No automated test runs were required because the change is documentation and a thin routing skill only.

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Codex Task