Add base_ref parameter to assign_copilot_to_issue tool

[Copilot]

Summary

Adds optional base_ref parameter to assign_copilot_to_issue tool and migrates from replaceActorsForAssignable to updateIssue GraphQL mutation with agentAssignment field.

Why

Enables specifying which branch Copilot should start from when working on an issue. Previously, only the default branch could be used.

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

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.

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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