chore: add codex skills implementation notebook

docs/CODEX-SKILLS-IMPLEMENTATION-NOTEBOOK.md

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+# Codex Skills-First Implementation Notebook
+
+## Metadata
+- Date: 2026-02-18
+- Branch: `feat/codex-skills-first-support`
+- Scope: Implement Codex support using skills-first approach (no deprecated custom prompts)
+- Requested by: User in this session
+
+## Goals
+1. Add Codex runtime option to installer (`--codex`) with proper config directory handling (`CODEX_HOME`, `~/.codex`, `./.codex`).
+2. Install GSD for Codex as skills under `skills/` (not prompts).
+3. Preserve GSD workflows by transpiling command files to Codex skill layout.
+4. Update path/command references for Codex usage patterns.
+5. Keep implementation installer-focused and avoid changing core workflow source files unless required.
+
+## Plan of Action
+1. Add runtime plumbing for Codex in `bin/install.js`.
+2. Add Codex skill transpiler and install/uninstall integration.
+3. Add Codex-specific reference rewriting for installed content.
+4. Update user-facing docs/help and finish-install guidance.
+5. Validate with iterative tests and smoke runs.
+
+## Work Log
+- [2026-02-18] Created implementation branch `feat/codex-skills-first-support`.
+- [2026-02-18] Initialized notebook with goals and implementation plan.
+
+## Test Log
+- None yet.
+
+## Commit Log
+- None yet.