docs(ai-skills): add scope and cross-agent install guidance (#45583)

## Summary

- Adds a note explaining that skills install at **project scope by
default** (committed to git, shared with team and cloud agents) and that
`--global` installs across all projects
- Mentions `--all` for users who work across multiple AI agents
- Cross-links the [Supabase agent
plugin](/docs/guides/getting-started/plugins) from both the agent skills
and MCP docs for users who want the MCP server and skills in a single
install

Closes
[AI-672](https://linear.app/supabase/issue/AI-672/add-other-plugins-to-docs-with-clearer-project-vs-global-level)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated AI skills installation guide with detailed instructions for
project-scoped, global, and multi-agent installation options
* Enhanced MCP setup guide with recommendation to install the Supabase
agent plugin for streamlined configuration
* Added direct link to the skills npm package for improved resource
accessibility
  * Refined compatibility information in AI skills guide

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Jeremias Menichelli <jmenichelli@gmail.com>

apps/docs/content/guides/getting-started/ai-skills.mdx

@@ -18,7 +18,11 @@ To install a specific skill from the repository:
 npx skills add supabase/agent-skills --skill SKILL_NAME
 ```
 
-Skills work with 18+ AI agents including Claude Code, GitHub Copilot, Cursor, Cline, and many others.
+Skills are installed at project scope by default, placing them in your repository so contributors and cloud agents all share the same setup. Pass `--global` to install across all your projects instead.
+
+Add skills for all detected agents at the same time by passing `--all`. See the [skills package](https://github.com/vercel-labs/skills) for more options.
+
+You can also install the agent skills together with the Supabase MCP server using the [Supabase agent plugin](/docs/guides/getting-started/plugins) for an agent-first workflow.
 
 ## Available skills
 
@@ -37,3 +41,4 @@ npx skills find QUERY
 - [Agent Skills Repository](https://github.com/supabase/agent-skills)
 - [Agent Skills Documentation](https://agentskills.io/home)
 - [Agent Skills Overview](https://platform.claude.com/docs/en/agents-and-tools/agent-skills/overview)
+- [skills npm package](https://github.com/vercel-labs/skills)

apps/docs/content/guides/getting-started/mcp.mdx

@@ -30,6 +30,8 @@ To verify the client has access to the MCP server tools, try asking it to query
 
 For curated, ready-to-use prompts that work well with IDEs and AI agents, see our [AI Prompts](/docs/guides/getting-started/ai-prompts) collection.
 
+Additionally, you can install Supabase agent skills alongside the MCP server, use the [Supabase agent plugin](/docs/guides/getting-started/plugins) for a combined one-step setup.
+
 ## Available tools
 
 The Supabase MCP server provides tools organized into feature groups. All groups except Storage are enabled by default. You can enable or disable specific groups using the [configuration panel above](#step-2-configure-your-ai-tool).