Add custom field filtering to list_issues

[kelsey-myers]

Summary

Extend list_issues so callers can filter issues by custom field values (single-select, text, number, date) alongside the existing label, state, and since filters.

Why

The IssueFilters.issueFieldValues argument is available in the GraphQL schema (gated by the issue_fields feature flag) and the existing list_issues tool returns field values on each issue, but there was no way to filter by them. Without this, agents have to fetch all issues and filter client-side which doesn't scale and is wasteful.

Stacked on top of #2466 — will retarget once that merges. Until then this PR shows both sets of commits.

What changed

• Added field_filters array parameter to list_issues. Each entry takes a field_name plus exactly one of single_select_value, text_value, number_value, or date_value.
• Threaded the filter through all four ListIssuesQuery* variants via a new issueFieldValues GraphQL variable. The variable is always sent (empty slice when no filters are supplied), the resolver treats an empty list as a no-op.
• Added parseFieldFilters helper that validates exactly one typed value per filter and propagates type errors uniformly.
• Set GraphQL-Features: issue_fields on outgoing requests via the existing GraphQLFeaturesTransport so the gated input type resolves while the flag is still rolling out. Safe to keep after rollout (no-op).
• Unit tests cover all four query variants, all four value types, validation paths, number_value: 0 (must be treated as set), and a regression guard that the GraphQL-Features header is sent.

MCP impact

• Tool schema or behavior changed

list_issues gains an optional field_filters parameter. Default behaviour (no field_filters) is unchanged.

Prompts tested (tool changes only)

• "List the latest P1 priority issues in github/issues" — single-select filter
• "List open issues in github/issues with the batch label" — labels-only path (regression)
• "List the latest 10 issues in github/issues" — no filters (regression)
• "List open issues updated since 2026-05-14" — since path (regression)
• Combined: field_filters + labels + since across all four query variants
• Each typed value field (single-select, text, number, date) exercised live against github/issues

Security / limits

• Auth / permissions considered

No new auth surface. Uses the same scopes as list_issues today. The GraphQL-Features header opts into a schema-level feature flag and does not bypass any authorization.

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

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

Docs

• Updated (README / docs / examples)

script/generate-docs regenerated README.md and the list_issues toolsnap.

[kelsey-myers]

Sample Input:

{
  "owner": "github",
  "repo": "test-repo",
  "state": "OPEN",
  "field_filters": [
    {
      "field_name": "Priority",
      "single_select_value": "P1"
    }
  ],
  "orderBy": "UPDATED_AT",
  "direction": "DESC",
  "perPage": 20
}

Sample Output:

{
  ...issue,
  "field_values": [
    { "field": "Priority",   "value": "P1"   },
    { "field": "Visibility", "value": "High" },
    { "field": "Impact",     "value": "High" },
    { "field": "Effort",     "value": "Low"  }
  ]
}

[leuasseurfarrelds247-arch]