Add a retro skill shared by Claude Code and Codex

Add a retro skill shared by Claude Code and Codex Retrospects on the current session: reconstructs the route taken, separates essential steps from waste, and derives the shortest route to the same verified endpoint. The body is tool-neutral and lives once under .config/ai beside working-preferences.md; each agent gets a link into the skills directory it scans. Linked per file rather than as a directory, because both ~/.claude/skills and ~/.codex/skills also hold skills the tools install themselves.