feat(cli): added skill ai

[duboolivier]

Description

Adds an agent skill for the ovhcloud CLI (#207): a compact, tool-agnostic
guide that lets AI coding agents drive the CLI correctly without inlining the
~900 generated command pages.

Architecture — one source, multiple entry points so every agent finds it:

• AGENTS.md (repo root) — the canonical guide (invocation pattern, global
  flags, golden rules + safety, links to the rest). Read by Codex, Cursor,
  Gemini CLI, Aider, Zed, etc.
• docs/agent-skill/ — load-on-demand bundle: references/ (commands map,
  flags, safety) and recipes/ (Public Cloud, account/auth, domains/DNS).
  Progressive disclosure keeps agent context small — the goal of the issue.
• .claude/skills/ovhcloud-cli/SKILL.md — Claude Code entry point, mirrors
  AGENTS.md.
• .github/copilot-instructions.md — GitHub Copilot entry point, points to
  AGENTS.md.

Design principles: read-only/safe by default (confirm before destructive or
billed operations), and rely on the CLI's own --help + -o json as the source
of truth rather than duplicating exhaustive command lists (avoids drift).

No CLI code is changed — this is documentation for AI agents only.

Fixes #207

Type of change

• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have commented my code
• I ran go mod tidy
• I have added tests that prove my fix is effective or that my feature works