docs: surface Codex CLI support

[0xGunDone]

This surfaces RTK's existing Codex CLI support in the docs.

What changes:

• add Codex setup to the main README
• add Codex installation and uninstall flows to INSTALL.md
• expand the Codex awareness file with a few more explicit RTK command examples and verification hints

Why:

• RTK already ships rtk init --codex and rtk init -g --codex
• the support exists in code, but it is easy to miss from the current docs
• this makes the current Codex story discoverable without over-claiming transparent hook-based rewriting

Behavior note:

• Codex support is documented here as awareness-based (AGENTS.md + RTK.md) and explicit rtk ... usage
• unlike Claude Code, it does not claim transparent shell rewrite hooks

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[pszymkowiak]

[w] wshm · Automated triage by AI

📊 Automated PR Analysis

📝 Type	docs
🟢 Risk	low

Summary

This PR adds documentation for RTK's existing Codex CLI support across README.md, INSTALL.md, and the Codex awareness file. It covers installation, usage, verification, and uninstall flows for Codex, clarifying that Codex support is awareness-based (via AGENTS.md/RTK.md files) rather than hook-based like Claude Code.

Review Checklist

• Tests present
• Breaking change
• Docs updated

---

Analyzed automatically by wshm · This is an automated analysis, not a human review.

[0xGunDone]

Maintainer note:

This PR is intentionally docs-only. RTK already ships Codex support in code via rtk init --codex / rtk init -g --codex and hooks/rtk-awareness-codex.md; the change here is to make that path discoverable in the README/install docs and to clarify that Codex support is awareness-based today, not transparent hook-based rewriting.

I verified the flow locally with rtk 0.31.0:

• rtk init -g --codex
• rtk init --show --codex

That produced ~/.codex/RTK.md plus an @RTK.md reference in ~/.codex/AGENTS.md as expected.