Support removing issue types

[zwick]

Summary

Allow consolidated issue_write and granular update_issue_type to remove an issue type with an empty-string input.

Why

Related: github/plan-track-agentic-org#193

What changed

• Preserve string-only schemas: omit consolidated type to leave it unchanged, use a non-empty string to set it, or use an empty string to clear it.
• Translate clear operations into one REST PATCH containing "type": null.
• Any update containing an explicit type clear bypasses MCP App form deferral because the unchanged form cannot preserve the empty-string sentinel. The complete call executes directly so co-submitted values are not dropped; REST fields share the PATCH and state transitions retain their existing GraphQL transport.
• Add focused exact-JSON coverage for omitted, set, clear, granular clear, UI-capable direct clear, and UI-capable clear combined with a completed state transition.

MCP impact

• No tool or API changes
• Tool behavior changed - empty issue type inputs remove the current type.
• New tool added

Prompts tested (tool changes only)

• "Set this issue type to Bug"
• "Remove the type from this issue"

Security / limits

• No security or limits impact - this uses the existing issue update permission and endpoint.
• 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

Lint & tests

• Linted locally with ./script/lint
• Tested locally with focused pkg/github tests

Focused tests cover omitted, non-empty set, empty-string clear, required granular input, exact PATCH JSON, UI-capable clear bypass, and a mixed clear-plus-state call using the existing REST/GraphQL transport split. Earlier live API validation used locally built stdio server calls without the MCP App form path; consolidated and granular tools both set and cleared a disposable issue type, read back type: null, and the issue was closed.

Docs

• Not needed
• Generated tool reference artifacts updated; generator unchanged

[Copilot]

Pull request overview

Adds empty-string support for clearing issue types in consolidated and granular issue tools.

Changes:

• Sends "type": null when clearing an issue type.
• Bypasses form deferral to preserve explicit clears.
• Adds exact request-body tests and updates generated documentation.

Show a summary per file

File	Description
README.md	Documents issue-type removal.
pkg/github/issues.go	Implements consolidated clear behavior.
pkg/github/issues_test.go	Tests set, omit, clear, and form bypass.
pkg/github/issues_granular.go	Implements granular clear behavior.
pkg/github/granular_tools_test.go	Tests granular clear and validation.
pkg/github/__toolsnaps__/update_issue_type.snap	Updates granular schema snapshot.
pkg/github/__toolsnaps__/issue_write.snap	Updates consolidated schema snapshot.
docs/insiders-features.md	Updates generated insiders documentation.
docs/feature-flags.md	Updates generated feature documentation.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 0
• Review effort level: Balanced

[zwick]

This was a little awkward, but a previous attempt to handle this null|string value was requiring a larger change to the Form to support a null value. Added this to bypass that deferment instead of changing how the form validates nullable data to keep this change focused

[alirezainanlosalehi7-lgtm]