[Docs]Add Custom Coding Agent for MeshKit

[1012Charan]

Description:
Add a specialized coding agent definition in .github/agents/meshkit-contributor.md to automate repository-specific workflows. This agent provides expert-level guidance on MeshKit’s internal utilities, specifically the errorutil registry management and strict module hygiene requirements

Desired State:
Error Utility: Automates the update and analysis cycles for error codes via make errorutil and make errorutil-analyze.

Dependency Management: Enforces the use of make tidy with built-in git diff validation to maintain go.mod integrity.

Build/Test Standards: Configures the agent to use repository-specific linting (make check) and unit test flags (-race, -covermode=atomic).

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[welcome]

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