projects: add item field support

[tmelliottjr]

Towards: #44

This PR adds an optional fields parameter to list_project_items and get_project_item tools, allowing users to specify which custom field IDs to include in responses. Previously, only the title field was returned by default.

The implementation includes a new ManageProjectItemsPrompt that provides step-by-step guidance for working with GitHub Projects, including discovering projects, viewing available fields, querying items with specific field values, and updating field values on project items.

The type system is updated to distinguish between projectV2Field (field definitions) and projectV2FieldValues (field values on items), ensuring proper serialization of field data. Comprehensive test coverage validates field selection functionality across both organization and user-owned projects.

[Copilot]

Pull Request Overview

This PR adds support for custom field selection in GitHub Projects V2 items, addressing issue #44. Users can now specify which field IDs to include when listing or retrieving project items, with only the title field returned by default if no fields are specified.

Key Changes:

• Added optional fields parameter to list_project_items and get_project_item tools for selecting specific custom fields
• Introduced ManageProjectItemsPrompt to guide users through project workflows including field discovery and updates
• Refactored type system to distinguish between field definitions (projectV2Field) and field values on items (projectV2ItemFieldValue)

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
pkg/github/tools.go	Registered the new ManageProjectItemsPrompt with the projects toolset
pkg/github/projects_test.go	Added test coverage for field parameter validation in list and get operations
pkg/github/projects.go	Implemented field selection logic, refactored options types, and added the ManageProjectItemsPrompt
pkg/github/minimal_types.go	Updated MinimalProjectItem to use projectV2ItemFieldValue type for fields
pkg/github/toolsnaps/list_project_items.snap	Updated schema snapshot to include fields parameter
pkg/github/toolsnaps/get_project_item.snap	Updated schema snapshot to include fields parameter
README.md	Documented the new fields parameter for both tools

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