Add comprehensive Copilot instructions for meshStack Terraform Provider

[Copilot]

This PR adds a comprehensive .copilot-instructions.md file to provide GitHub Copilot with detailed context about the meshStack Terraform Provider repository structure, development patterns, and API integration.

What's included

The instructions file covers:

• Repository overview: Purpose as the official meshStack Terraform Provider integrating with meshStack's meshObject API
• Architecture details: Standard Terraform provider structure with authentication, resource management, and API client patterns
• Directory structure: Detailed explanations of key directories (/internal/provider/, /client/, /docs/, /examples/)
• Development patterns: Resource implementation patterns, API client conventions, and schema structures
• Build and testing: Local development setup, testing procedures, and documentation generation workflow
• Supported resources: Complete list of managed resources including building blocks, projects, workspaces, tenants, and tag definitions
• Troubleshooting guide: Common issues and debugging approaches

Key information for Copilot

The instructions provide essential context about:

• Integration with meshStack's meshObject API (/api/meshobjects endpoints)
• API documentation reference: https://docs.dev.meshcloud.io/api/index.html#mesh_objects
• Authentication using JWT tokens with automatic refresh
• Standard meshObject structure (api_version, kind, metadata, spec, status)
• Resource versioning strategy (building blocks v1/v2, tenants v4)
• Development workflow and common tasks

This will enable Copilot to provide more accurate and contextually relevant assistance to developers working on the provider.

Fixes #49.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[JohannesRudolph]

• @j0g3sc you will need to add documentation URLs to the allow list at https://github.com/meshcloud/terraform-provider-meshstack/settings/copilot/coding_agent

[j0g3sc]

@JohannesRudolph Thanks for the hint.
This PR is to get a first rough draft for copilot instructions. I let it generate it itself and then discarded a bit.

[JohannesRudolph]

lgtm for a first pass, will be refined based on real world performance anyway

[JohannesRudolph]

Suggested change

	- **API Documentation**: https://docs.dev.meshcloud.io/api/index.html#mesh_objects
	- **API Documentation**: https://docs.meshcloud.io/api/index.html#mesh_objects

[JohannesRudolph]

d: we also publish to opentofu registry

[JohannesRudolph]

d: not sure if this is wasted context (auth is already codified in the provider)

[JohannesRudolph]

a more useful explanation here would be the key concept of workspace and that most resources live in a workspace, and that API tokens are also usually scoped to a single workspace