Promote issue fields and deprecate legacy issue write tool

[reneexeener]

Summary

This PR removes the remote_mcp_issue_fields feature flag, making issue fields and related functionality (e.g. field_filters support in issue tools) available by default, and updates documentation and tests to reflect this change.

Why

• Fixes https://github.com/github/plan-track-agentic-toolkit/issues/317

What changed

Theme	Change Description	File(s) / Reference(s)
Feature Flag Removal	Remove the FeatureFlagIssueFields (remote_mcp_issue_fields) feature flag and its usage from the codebase and allowed flags list.	pkg/github/feature_flags.go [1] [2]
Documentation Cleanup	Remove conditional documentation for remote_mcp_issue_fields and always document issue field support in issue tools.	docs/feature-flags.md [1] [2] docs/insiders-features.md [3] [4]
Tool Schema Updates	Remove feature-flagged tool schema snapshots for issue fields (*_ff_remote_mcp_issue_fields.snap).	pkg/github/__toolsnaps__/issue_write_ff_remote_mcp_issue_fields.snap [1] pkg/github/__toolsnaps__/list_issues_ff_remote_mcp_issue_fields.snap [2]
Default Tool Expansion	Add issue_fields and field_filters parameters to the default tool schemas and documentation.	README.md [1] [2] [3] pkg/github/__toolsnaps__/issue_write.snap [4] pkg/github/__toolsnaps__/list_issues.snap [5]
Test Adjustments	Update tests to use a different feature flag (FeatureFlagFileBlame) for CSV output gating, reflecting the removal.	pkg/github/csv_output_test.go

These changes streamline the codebase by making issue field support a standard feature, reducing complexity and improving documentation clarity.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

• "suggest appropriate labels and issue fields for the issue [issue URL]"
  • Confirmed that labels and issue fields were successfully applied

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

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

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

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

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

This PR removes the remote_mcp_issue_fields feature flag so Issues 2.0 custom fields support (listing fields, filtering by fields, and enriching issue results with field_values) is available by default, and it deletes the legacy tool variants that existed only for the flag-off behavior.

Changes:

• Removed the remote_mcp_issue_fields feature flag and legacy list_issues / issue_write variants that depended on it.
• Promoted issue fields support into the default tool schemas (issue_fields on issue_write, field_filters on list_issues) and updated tool snapshots accordingly.
• Updated documentation and tests to reflect the always-on issue fields behavior.

Show a summary per file

File	Description
README.md	Documents list_issue_fields and the now-default issue_fields / field_filters parameters.
pkg/github/ui_tools.go	Makes UI issue-fields retrieval unconditional (no feature-flag gate).
pkg/github/ui_tools_test.go	Updates UI tool tests to validate successful issue-fields fetch.
pkg/github/tools.go	Removes registration of legacy issue tools from the full tool list.
pkg/github/minimal_types.go	Deletes legacy minimal conversion helpers used by the flag-off list_issues variant.
pkg/github/issues.go	Removes legacy GraphQL types and tool variants; always supports field filters/enrichment.
pkg/github/issues_test.go	Updates tests and toolsnap assertions to match the default (always-on) issue fields behavior.
pkg/github/issue_fields.go	Un-gates list_issue_fields so it’s always registered.
pkg/github/feature_flags.go	Deletes the remote_mcp_issue_fields flag constant and removes it from allowed/insiders lists.
pkg/github/csv_output_test.go	Switches the CSV gating test to use FeatureFlagFileBlame after flag removal.
pkg/github/toolsnaps/list_issues.snap	Adds field_filters to the default list_issues schema snapshot.
pkg/github/toolsnaps/list_issues_ff_remote_mcp_issue_fields.snap	Removes the flag-specific list_issues snapshot.
pkg/github/toolsnaps/issue_write.snap	Adds issue_fields to the default issue_write schema snapshot.
pkg/github/toolsnaps/issue_write_ff_remote_mcp_issue_fields.snap	Removes the flag-specific issue_write snapshot.
docs/insiders-features.md	Removes remote_mcp_issue_fields section; documents issue fields as default.
docs/feature-flags.md	Removes remote_mcp_issue_fields section; documents issue fields as default.

Copilot's findings

• Files reviewed: 16/16 changed files
• Comments generated: 0