feat(codex): add readme codex skill

.codex/skills/readme/SKILL.md

@@ -0,0 +1,64 @@
+---
+name: readme
+description: Write and update README.md and README-ko.md for the nix-darwin repository. Use when asked to create new repository documentation, refresh outdated setup/usage instructions, summarize the current project structure, or produce Korean README content.
+---
+
+# README Writer
+
+## Overview
+
+Generate high-quality README files for this repository based on current code and
+configuration. Create missing READMEs or update existing ones while preserving
+useful, project-specific content.
+
+## Workflow
+
+1. Inspect repository facts before writing. Use targeted reads of `flake.nix`,
+   `Justfile`, `home/`, `modules/`, and `secrets/` plus recent git history. Do
+   not invent commands, tools, or directory names.
+
+2. Select target file and language. Default target is `README.md` in English. If
+   user asks for Korean or explicitly requests `README-ko.md`, write Korean
+   content to `README-ko.md`. If target is ambiguous, ask one concise
+   clarification question.
+
+3. Draft content before mutating files. Always show a draft summary (or full
+   draft when requested) and request explicit confirmation before writing. Use
+   concise, actionable prose and include concrete commands that work in this
+   repository.
+
+4. Create or update safely. If the target README does not exist, create it. If
+   it exists, retain useful custom sections and update stale technical details.
+   Avoid deleting user-authored content unless it is clearly obsolete and
+   replaced by accurate content.
+
+5. Validate the result. Re-check that all commands and paths referenced in the
+   README exist in the repository. Ensure headings are clear, markdown is valid,
+   and tone is professional.
+
+## Recommended README Structure
+
+- Title and short description
+- Prerequisites (Nix/macOS assumptions when relevant)
+- Repository layout (`home/`, `modules/`, `secrets/`, root files)
+- Key commands (`just darwin`, `just darwin-debug`, `just fmt`, update/cleanup
+  commands)
+- Configuration and secrets notes (`.sops.yaml`, `secrets/*.yaml`)
+- Common workflows (apply config, update flake inputs, debug build issues)
+
+Adapt section names if user requests a different format, but keep the content
+repository-specific.
+
+## Output Rules
+
+- Prefer concise explanations over long tutorials.
+- Keep examples runnable from repository root.
+- Use Markdown headings and fenced code blocks for commands.
+- Do not include placeholders like "TODO" in final README output.
+- For bilingual requests, keep language consistent per file (no mixed-language
+  sections unless requested).
+
+## Confirmation Requirement
+
+Never write or overwrite `README.md` or `README-ko.md` without explicit user
+confirmation in the current conversation.