Add contributing guidelines to prevent seed.json PRs (#1079)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
We've been receiving numerous PRs from authors attempting to publish
their MCP server by adding entries to `data/seed.json`. This is not the
correct approach as servers should be published using the
`mcp-publisher` CLI tool.

Many of these PRs appear to be AI-generated so the existing docs aren't
being surfaced effectively. This PR adds explicit guidance in the files
that AI coding assistants actually read:
- `CONTRIBUTING.md` - shown by GitHub when opening new PRs; widely
ingested by AI tools
- `.github/copilot-instructions.md` - read by GitHub Copilot
- `CLAUDE.md` - read by Claude Code and Claude on github.com

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->

## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

.github/copilot-instructions.md

@@ -0,0 +1,13 @@
+# Copilot Instructions for MCP Registry
+
+## Important: Publishing MCP servers
+
+The `data/seed.json` file is seed data for local development only. Do NOT create pull requests that add or modify server entries in `data/seed.json` to publish a server.
+
+To publish an MCP server to the registry, use the `mcp-publisher` CLI tool. See `docs/modelcontextprotocol-io/quickstart.mdx` for instructions.
+
+## Development
+
+- Use `make` targets where possible (run `make help` to see available targets)
+- Run `make check` to run lint, unit tests, and integration tests
+- Run `make dev-compose` to start the local development environment