Add base_ref parameter and GraphQL-Features header support to assign_copilot_to_issue tool

[Copilot]

Summary

Adds optional base_ref parameter to assign_copilot_to_issue tool, migrates from replaceActorsForAssignable to updateIssue GraphQL mutation with agentAssignment field, and implements GraphQL-Features header support for the non-GA agent assignment API.

Why

Enables specifying which branch Copilot should start from when working on an issue. Previously, only the default branch could be used. The agent assignment API is not yet GA and requires the GraphQL-Features header to function.

Related: community/community#173575 (comment)

What changed

• Added optional base_ref parameter to tool schema (defaults to repository default branch if omitted)
• Replaced replaceActorsForAssignable mutation with updateIssue mutation using agentAssignment field
• Added AgentAssignmentInput and UpdateIssueInput structs for new GraphQL mutation structure
• Updated query to fetch repository ID alongside issue ID in single call
• Updated all test cases to use new mutation structure, added test for base_ref parameter
• Implemented context-based GraphQL feature flag system for passing features to HTTP transport layer
• Modified bearerAuthTransport to read GraphQL features from context and add GraphQL-Features header
• Tool now sends GraphQL-Features: issues_copilot_assignment_api_support header for agent assignment API

MCP impact

• Tool schema or behavior changed
  Tool now accepts optional base_ref parameter. GraphQL mutation changed from replaceActorsForAssignable to updateIssue with agentAssignment field containing baseRef, customAgent, customInstructions, and targetRepositoryId. Requests now include the GraphQL-Features: issues_copilot_assignment_api_support header required by the non-GA API.

Prompts tested (tool changes only)

• "Assign copilot to issue Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 in owner/repo starting from feature-branch"
• "Assign copilot to issue Bump go-github to v72.0.0 #456 in owner/repo" (uses default branch)

Security / limits

• No security or limits impact

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)

Original prompt

The assign issue to copilot tool should now enable base branch specification:

<<<
This should be possible now we've opened up the GraphQL endpoint - however we might want to wait until the FF has been promoted so that we don't have to send FFs in the GraphQL Request Header.

We used the ref for the branch:

gh api graphql -f query='mutation() {
updateIssue(input:{
id:$ISSUE_ID,
assigneeIds:["BOT_kgDOC9w8XQ"],
agentAssignment:{ baseRef: "main", customAgent: "", customInstructions: "Add a cat emoji to the readme", targetRepositoryId: $REPO_ID }}){issue{url number id}}}' -H 'GraphQL-Features: issues_copilot_assignment_api_support'
So it should be simpler - not providing a baseRef should mean that the base branch is used.

Don't worry about the feature flag, just look at implementing the correct mutation with optional argument.

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[SamMorrowDrums]

@copilot add in GraphQL-Features: issues_copilot_assignment_api_support feature to this tool please, so the API actually works, it's not GA yet so requires the feature to be provided. Related discussion is here: https://github.com/orgs/community/discussions/173575

This should use the standard github GraphQL feature support.

[Copilot]

@copilot add in GraphQL-Features: issues_copilot_assignment_api_support feature to this tool please, so the API actually works, it's not GA yet so requires the feature to be provided. Related discussi...

Added GraphQL-Features header support in commit ced0c4c. The tool now sends the GraphQL-Features: issues_copilot_assignment_api_support header when calling the updateIssue mutation. The implementation uses context-based feature flags that are automatically added by the HTTP transport layer.