Add list_issue_fields tool

[kelsey-myers]

Summary

Adds a list_issue_fields tool that returns field definitions (name, type, options) for a GitHub org or repo. Picks up where #2438 left off, with some direction changes since.

Why

Agents need to discover available custom fields before they can set field values on issues. Without this, they're guessing field names and types.

Fixes https://github.com/github/plan-track-agentic-toolkit/issues/38

What changed

• Added issue_fields.go with list_issue_fields tool — queries repository.issueFields or organization.issueFields via GraphQL depending on whether repo is passed
• repo is optional: passing owner+repo returns fields inherited from the org; passing owner alone returns org-level fields directly
• Scopes: repo or read:org (either suffices, matching list_issue_types approach)
• Added WithGraphQLFeatures(ctx, "issue_fields") so the query hits the feature-flagged field on the monolith
• Added tests for both paths, scope assertions, toolsnap
• Updated README

MCP impact

• New tool added

list_issue_fields — read-only tool returning field definitions (id, name, data_type, visibility, options for single_select). Requires repo or read:org scope.

Prompts tested

• "List all issue fields for the github org"
• "What custom fields are available for issues in the github/github-mcp-server repo?"

Security / limits

• Auth / permissions considered

repo or read:org required. User-owned repos return [] (no error) — monolith returns null for non-org owners, which resolves to an empty list naturally.

Tool renaming

• I am not renaming tools as part of this PR

Note: the original PR used list_org_issue_fields — renamed to list_issue_fields since it now covers both repo and org.

Lint & tests

• Linted locally with lint
• Tested locally with test

Docs

• Updated (README / docs / examples)

[Copilot]

Pull request overview

Adds a new read-only MCP tool to list organization-level issue field definitions (including single-select options) so agents can discover valid custom fields before creating/updating issues.

Changes:

• Implemented list_org_issue_fields tool calling GET /orgs/{org}/issue-fields, with 404 returning an empty list.
• Added unit tests covering success and common error scenarios.
• Registered the tool, added a toolsnap, and updated README tool docs.

Show a summary per file

File	Description
README.md	Documents the new list_org_issue_fields tool and required scopes.
pkg/github/tools.go	Registers the new tool in the issues toolset.
pkg/github/issue_fields.go	Implements the tool and response types for org issue fields/options.
pkg/github/issue_fields_test.go	Adds unit tests for the new tool behavior.
pkg/github/toolsnaps/list_org_issue_fields.snap	Captures the tool schema snapshot for regression checking.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[kelsey-myers]

Returns:

[
 {
    "id": "IFSS_kgAD",
    "name": "Visibility",
    "description": "Who can view or is impacted by the issue.",
    "data_type": "SINGLE_SELECT",
    "visibility": "ORG_ONLY",
    "options": [
      {
        "id": "IFSSO_kgAJ",
        "name": "High",
        "color": "RED",
        "priority": 0
      },
      {
        "id": "IFSSO_kgAK",
        "name": "Medium",
        "color": "YELLOW",
        "priority": 0
      },
      {
        "id": "IFSSO_kgAL",
        "name": "Low",
        "color": "GREEN",
        "priority": 0
      }
    ]
  },
...
]

[Copilot]

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 5

[kelsey-myers]

Benchmarks: https://github.com/github/copilot-offline-eval/pull/4637/changes