Add agent-first onboarding quickstart

[manojbajaj95]

Goal

Add an agent-first onboarding path so someone can point an AI coding agent at Authsome and get authenticated tooling without needing to understand Authsome first.

The quickstart should make Authsome feel like invisible local credential plumbing for agents: install/login once, then agents can run with fresh provider credentials through authsome run, authsome export, or provider-specific setup guidance.

Research notes

Reference styles:

• Agent Skills / agent marketplace style: concise Getting Started, What is included, and skill-oriented instructions that agents can follow directly. Example: https://github.com/agentskills/agentskills and marketplace listings that expose a SKILL.md-style entry point.
• Oh My OpenCode README style: explicitly includes agent-facing copy like “hand this doc to an agent,” “for LLM agents,” and a minimal magic-word / copy-paste path before deeper docs. Official repo reference: https://github.com/code-yeongyu/oh-my-opencode
• Authsome already has the product framing in README.md (“Local auth for AI agents”), but the Quick Start is still human/CLI-oriented and assumes the reader knows they are choosing Authsome.

Proposed scope

Add a short onboarding section, likely near the top of README.md, plus a deeper docs page if needed, with:

• A “For agents” or “Give this to your agent” quickstart that is copy-pasteable into Claude Code, Codex, OpenCode, Cursor, etc.
• A minimal path for the agent to detect/install/use Authsome, e.g. check authsome, install if missing, run authsome doctor, login required providers, then run the target command with authsome run -- ....
• Example prompts that hide Authsome from the end user, such as: “Set up this project so the agent can call GitHub/OpenAI APIs locally without hardcoded tokens.”
• Provider examples for the most likely agent use cases: GitHub, OpenAI, Linear.
• A clear explanation that credentials stay local and encrypted, but keep this below the quickstart so the first path remains action-oriented.
• Optional agent skill artifact, e.g. docs/agent-quickstart.md or a SKILL.md-style snippet that downstream projects can paste into their own agent instructions.

Acceptance criteria

• New users can get from zero to an agent using Authsome in under 5 commands or one copy-paste agent prompt.
• The first-screen README path is agent-first, not only a human CLI checklist.
• The docs include both interactive local setup and headless/SSH-friendly setup (device_code).
• The examples avoid encouraging long-lived plaintext environment tokens as the primary path.
• The quickstart works even if the user does not already know Authsome terminology.