docs: add Codex PreToolUse adapter guide

[xuegaoge]

Summary

• Adds docs/usage/CODEX_PRETOOLUSE_ADAPTER.md, a focused guide for installing an optional Codex Desktop/CLI PreToolUse adapter.
• Keeps hooks/codex/README.md as the hook-directory overview and links it to the new usage guide.
• Provides an idempotent quick-install snippet that reuses an existing RTK install when rtk gain works, and installs RTK only when missing or incorrect.

Why

rtk init --codex currently gives Codex prompt-level awareness through AGENTS.md and RTK.md, but it does not provide a native Codex hook. The documented adapter lets Codex rewrite Bash commands at tool-call time while preserving normal pass-through behavior when RTK has no rewrite.

Validation

• Ran git diff --check HEAD~1..HEAD.
• Executed the quick-install snippet twice against a temporary CODEX_HOME and verified it remained idempotent with one PreToolUse entry.
• Verified the generated hook compiles with python3 -m py_compile.
• Simulated a Codex Bash payload and confirmed git status rewrites to rtk git status.
• Simulated a non-Bash payload and confirmed the hook emits zero bytes.

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[xuegaoge]

Closing in favor of #2069, which combines the native Codex plugin approach from #2063 with the safer tool_input-preserving behavior and user-facing setup guide from this PR.