Add base_ref support to assign_copilot_to_issue tool

- Add optional base_ref parameter to tool schema
- Change from replaceActorsForAssignable to updateIssue mutation with agentAssignment
- Add AgentAssignmentInput and UpdateIssueInput structs for new GraphQL mutation
- Update all tests to use new mutation structure
- Add test case for base_ref functionality
- Update toolsnaps and documentation

Co-authored-by: SamMorrowDrums <[email protected]>

Author: Copilot

README.md

@@ -751,6 +751,7 @@ The following sets of tools are available:
 
 - **assign_copilot_to_issue** - Assign Copilot to issue
   - **Required OAuth Scopes**: `repo`
+  - `base_ref`: Git reference (e.g., branch) that the agent will start its work from. If not specified, defaults to the repository's default branch (string, optional)
   - `issue_number`: Issue number (number, required)
   - `owner`: Repository owner (string, required)
   - `repo`: Repository name (string, required)