Include custom issue field values in list_issues response

[kelsey-myers]

Summary

Adds Issues 2.0 custom field values (Priority, Visibility, Impact, etc.) to each issue returned by list_issues, exposed on MinimalIssue as field_values: [{field, value}].

Why

Issues 2.0 introduced custom fields (single-select, number, text, date) that are core to how teams triage and prioritise. Without them in the MCP response, an agent can list issues but has no way to see — or reason about — the values teams actually filter and sort by in the GitHub UI.

Fixes https://github.com/github/plan-track-agentic-toolkit/issues/121

What changed

• IssueFragment now selects issueFieldValues(first: 25) (matches monolith IssueField::ORGANIZATION_ISSUE_FIELDS_LIMIT).
• New IssueFieldRef type with a Name() method, resolving the field name across the 4-variant IssueFields union.
• New IssueFieldValueFragment union fragment over IssueFieldDate|Number|SingleSelect|Text Value.
• MinimalIssue gains FieldValues []MinimalIssueFieldValue (omitempty); each entry carries field, value, and a forward-compat values []string for the upcoming IssueFieldMultiSelectValue (see github/github#430793).
• fragmentToMinimalIssueFieldValue converts the fragment to the minimal shape.
• Tests cover both label-filtered and non-label-filtered query variants.

Live validation

Smoke-tested against production github.com via a local build of this server, against github/issues:

{ "number": 21640, "field_values": [
  {"field":"Priority","value":"P2"},
  {"field":"Visibility","value":"Medium"},
  {"field":"Impact","value":"Medium"},
  {"field":"Effort","value":"Medium"}
]}

MCP impact

• No tool or API changes
• Tool schema or behavior changed
  • list_issues response now includes an optional field_values array per issue. Additive only, omitted for issues without custom field values, so existing consumers aren't broken.
• New tool added

Prompts tested (tool changes only)

• "List the open issues in github/issues and tell me which ones are P1." — agent can now answer from the response without follow-up tool calls.
• "What's the priority breakdown of open issues in github/issues?"

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered
  • first: 25 caps the per-issue field array at the same limit the dotcom monolith enforces (ORGANIZATION_ISSUE_FIELDS_LIMIT). No new auth surface — the values are scoped to the same repo permission the existing query already requires.

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
  • Schema is auto-generated from tool definitions; no README changes required for an additive response field.
• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR extends the list_issues tool’s issue output (MinimalIssue) to include GitHub Issues 2.0 custom issue field values (e.g., Priority/Impact) by selecting issueFieldValues(first: 25) in the GraphQL query and flattening the union results into a minimal {field, value} shape.

Changes:

• Add field_values (array of {field, value, values}) to MinimalIssue and populate it when converting GraphQL fragments.
• Introduce GraphQL fragment structs (IssueFieldRef, IssueFieldValueFragment) to read field names/values across the union types.
• Update list_issues tests to include issueFieldValues in mocked responses and expected query strings.

Show a summary per file

File	Description
pkg/github/minimal_types.go	Adds MinimalIssue.FieldValues and conversion logic to flatten custom field values into the minimal response shape.
pkg/github/issues.go	Extends the IssueFragment GraphQL selection with issueFieldValues(first: 25) and adds supporting fragment types for union decoding.
pkg/github/issues_test.go	Updates mocked GraphQL responses/expected queries and asserts field_values flattening for list_issues.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 1