feat: Add create org invitation tool

[issei-m]

I've added only the create-invitation, since that’s the only one I need at the moment.
And this is the first pull-req for this repository, so I'm not sure whether other tools for org should be implemented as well. (if so, I can do that)

[Copilot]

Pull Request Overview

This PR introduces a new GitHub organization invitation tool that allows creating invitations for users to join an organization through the MCP server. The tool supports inviting users by either GitHub user ID or email address and includes comprehensive validation and error handling.

• Added CreateOrgInvitation function to create organization invitations via GitHub API
• Integrated the new tool into the existing toolset framework as a write operation
• Comprehensive test coverage for various scenarios including success cases, validation errors, and API failures

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
pkg/github/tools.go	Integrates the new CreateOrgInvitation tool into the orgs toolset as a write operation
pkg/github/orgs.go	Implements the core CreateOrgInvitation function with parameter validation and GitHub API integration
pkg/github/orgs_test.go	Provides comprehensive test coverage for the CreateOrgInvitation functionality
pkg/github/toolsnaps/create_org_invitation.snap	Contains the tool definition snapshot for testing validation

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