docs: add multi-ecosystem plugin installation instructions

[fortuna]

Provides explicit installation commands for setting up Outline Skills as a plugin or extension across Codex, Claude Code, and Gemini CLI.

[greptile-apps]

Greptile Summary

This PR expands the README.md Install section with explicit per-ecosystem commands for Codex, Claude Code, and Gemini CLI.

• The Codex (npx skills add) and Gemini CLI (gemini extensions install) commands match their respective documented CLIs and are correct.
• The Claude Code instruction uses /plugin install <github-url>, which does not match the documented two-step workflow (/plugin marketplace add → /plugin install <name>@<marketplace>); a bare HTTPS URL is interpreted as a pointer to a hosted marketplace.json, not a GitHub repository clone, so the command will fail for users.

Confidence Score: 3/5

Safe to merge only after fixing the Claude Code install command — as written, users following the instructions will hit an error.

The Claude Code section instructs users to run /plugin install with a bare HTTPS URL, but Claude Code treats such a URL (without a .git suffix) as a pointer to a hosted marketplace.json rather than a GitHub repository clone. The actual documented workflow is a two-step process: /plugin marketplace add to register the repo, then /plugin install name@marketplace to install. Anyone following the README as written will get a confusing error. The Gemini CLI and Codex commands are correct.

README.md — specifically the Claude Code install block

Important Files Changed

Filename	Overview
README.md	Adds multi-ecosystem installation instructions; Gemini CLI and Codex commands are correct, but the Claude Code /plugin install command uses incorrect syntax that will fail at runtime

Sequence Diagram

sequenceDiagram
    participant User
    participant Codex as Codex CLI (npx skills)
    participant ClaudeCode as Claude Code
    participant GeminiCLI as Gemini CLI
    participant GitHub as GitHub Repo

    User->>Codex: npx skills add url
    Codex->>GitHub: Fetch plugin from URL
    GitHub-->>Codex: Plugin installed

    User->>ClaudeCode: /plugin install url (current - incorrect)
    ClaudeCode-->>User: Treats URL as marketplace.json link, fails

    User->>ClaudeCode: /plugin marketplace add OutlineFoundation/outline-skills
    ClaudeCode->>GitHub: Clone repo, read marketplace.json
    GitHub-->>ClaudeCode: Marketplace registered
    User->>ClaudeCode: "/plugin install outline-skills@outline-skills"
    ClaudeCode-->>User: Plugin installed

    User->>GeminiCLI: gemini extensions install url
    GeminiCLI->>GitHub: Fetch extension from URL
    GitHub-->>GeminiCLI: Extension installed

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[greptile-apps]

The /plugin install command expects a <plugin-name>@<marketplace> argument — not a raw HTTPS URL. An HTTPS URL without a .git suffix is treated by Claude Code as a direct link to a hosted marketplace.json, not as a GitHub repository. The correct flow is to first register the repo as a marketplace with /plugin marketplace add, then install the named plugin from it. The repository will also need a .claude-plugin/marketplace.json file (in addition to plugin.json) for the marketplace discovery step to work.

Suggested change

	To install as a Claude Code plugin, run inside your interactive Claude Code session:
	```shell
	/plugin install https://github.com/OutlineFoundation/outline-skills
	```
	To install as a Claude Code plugin, run inside your interactive Claude Code session:
	```shell
	/plugin marketplace add OutlineFoundation/outline-skills
	/plugin install outline-skills@outline-skills

[fortuna]

Converting to draft. I want to add the marketplace wrappers. And perhaps some screenshots.